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Patent 2030607 Summary

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(12) Patent: (11) CA 2030607
(54) English Title: METHOD OF MAINTAINING AN ESTABLISHED CONNECTION IN A MOBILE RADIO SYSTEM COMPRISING BOTH ANALOG AND DIGITAL RADIO CHANNELS
(54) French Title: METHODE POUR MAINTENIR UNE CONNEXION ETABLIE DANS UN SYSTEME RADIO MOBILE COMPORTANT A LA FOIS DES CANAUX RADIO ANALOGIQUES ET NUMERIQUES
Status: Deemed expired
Bibliographic Data
(52) Canadian Patent Classification (CPC):
  • 325/31
(51) International Patent Classification (IPC):
  • H04B 7/26 (2006.01)
  • H04W 36/14 (2009.01)
  • H04L 1/20 (2006.01)
  • H04L 25/02 (2006.01)
  • H04Q 7/38 (2006.01)
(72) Inventors :
  • DAHLIN, JAN ERIK AKE STEINER (Sweden)
  • UDDENFELDT, JAN ERIK (Sweden)
(73) Owners :
  • TELEFONAKTIEBOLAGET LM ERICSSON (Sweden)
(71) Applicants :
(74) Agent: MARKS & CLERK
(74) Associate agent:
(45) Issued: 2001-08-14
(86) PCT Filing Date: 1990-03-28
(87) Open to Public Inspection: 1990-11-01
Examination requested: 1997-02-10
Availability of licence: N/A
(25) Language of filing: English

Patent Cooperation Treaty (PCT): Yes
(86) PCT Filing Number: PCT/SE1990/000199
(87) International Publication Number: WO1990/013187
(85) National Entry: 1990-11-22

(30) Application Priority Data:
Application No. Country/Territory Date
8901385-8 Sweden 1989-04-17
371126 United States of America 1989-06-26

Abstracts

English Abstract




The invention relates to a method in a mobile radio system having
both digital and analog radio channels. An object of the invention
is that a connection established on a digital radio channel often
shall be possible to maintain even when the time dispersion on
available digital radio channels exceeds the designed maximum time
dispersion of equalizers in receiving stations. According the
invention equalizers in the receivers of the system may be
designed for a smaller maximum time dispersion than the total time
dispersion the whole system shall be capable to handle. Instead
handover is made to an other channel when the time dispersion of
a used digital channel tends to become too big. When an other
digital radio channel with low time dispersion is available
handover is preferably done to this channel. Alternatively or in
the absence of such a digital radio channel handover is performed
to an analog radio channel.


Claims

Note: Claims are shown in the official language in which they were submitted.



28
We claim:
1. In a multi-channel mobile radio system comprising base
stations and mobile stations designed both for digital
transmission of information using digitally modulated radio
signals on a digital radio channel and for analog
transmission of information using analog-modulated radio
signals on an analog radio channel, which mobile stations
comprise equalizers designed for a particular maximum time
dispersion of digitally modulated radio signals, a method of
maintaining a connection using a digital radio channel
comprising the steps of:
estimating the time dispersion of at least the digital
radio channel used by a mobile radio station for the
connection; and
when the time dispersion of the digital radio channel is
estimated to be too big, performing handover to an analog
radio channel of the system assumed to have sufficiently good
radio propagation properties if such an analog radio channel
is available to the mobile station.
2. A method according to claim 1, comprising the further
steps of:
estimating the time dispersion of at least one possible
digital radio channel available to the mobile station during
the time the connection continues on the analog radio
channel; and


29
when the time dispersion of a possible digital radio
channel available to the mobile station is estimated to be
sufficiently small, performing handover to this available
digital radio channel.
3. In a multichannel mobile radio system comprising base
stations and mobile stations designed both for digital
transmission of information using digitally modulated radio
signals on a digital radio channel and for analog
transmission of information using analog-modulated radio
signals on an analog radio channel, which mobile stations
comprise equalizers designed for a particular maximum time
dispersion of digitally modulated radio signals, a method of
maintaining a connection comprising the steps of:
estimating the time dispersion of at least a first digital
radio channel used by a mobile radio station for an ongoing
connection;
estimating the time dispersion of at least a second
possible digital radio channel to the mobile station at least
when the time dispersion of the first digital radio channel
is estimated to be too big;
performing handover to the second digital radio channel
when the time dispersion of the first radio channel is
estimated to be too big while the time dispersion of the
second radio channel is estimated to be sufficiently low; and
in the absence of any other available digital radio
channel to the mobile station having sufficiently low time
dispersion when the time dispersion of the first radio
channel is estimated to be too high, performing handover to


30
an analog radio channel estimated to have sufficiently good
radio propagation propertied if such an analog radio channel
is available to the mobile station.
4. A method according to claim 3, comprising the further step
of:
estimating the time dispersion of at least one possible
digital radio channel available to the mobile station during
the time the connection continues on the analog radio
channel; and
when the time dispersion of a possible digital radio
channel available to the mobile station is estimated to be
sufficiently low, performing handover to this available
digital radio channel.
5. In a multiple channel mobile radio system comprising base
stations and mobile stations designed both for digital
transmission of information using digitally modulated radio
signals on any of a plurality of digital radio channels and
for analog transmission of information using analog-modulated
radio signals on any of a plurality of analog radio channels,
which mobile stations comprise equalizers designed for a
particular maximum time dispersion of digitally modulated
radio signals, a method of maintaining a connection
comprising the steps of:
estimating at a mobile station having an ongoing
connection on a digital radio channel the time dispersion
indirectly by measuring signal level and symbol error rate of
the used digital radio channel used by that mobile radio
station for the ongoing connection;



31
estimating indirectly the time dispersion of at least a
second possible digital radio channel available to the mobile
station at least when the symbol error rate of the used
digital radio channel tends to be too high without the signal
level on the used digital radio channel tending to be too
low;
performing handover to the second digital radio channel
when the symbol error rate of the used radio channel is
estimated to be too high while the time dispersion of the
second radio channel is estimated to be sufficiently low; and
in the absence of any other digital radio channel
available to the mobile station having time dispersion
estimated to be sufficiently low when the symbol error rate
of the used digital radio channel is estimated to be
sufficiently high, performing handover to an analog radio
channel estimated to have sufficiently good radio propagation
properties if such an analog radio channel is available to
the mobile station.
6. A method according to claim 5, comprising the further
steps of:
estimating the time dispersion of at least one possible
digital radio channel to the mobile station during the time
the connection continues on the analog radio channel after
handover; and
when the time dispersion of an available digital radio
channel to the mobile station is estimated to be sufficiently
low, performing handover to this available digital radio
channel.


32
7. In a multi-channel mobile radio system comprising a base
station and a mobile station designed both for digital
transmission of information using digitally modulated radio
signals on a digital radio channel and for analog
transmission of information using analog-modulated radio
signals on an analog radio channel, which mobile station
comprises an equalizer designed for a particular maximum time
dispersion of digitally modulated radio signals, a method of
maintaining a connection started on the digital radio channel
comprising the steps of:
estimating time dispersion of the digital radio channel used
by the mobile radio station for the connection; and
comparing the estimated time dispersion with criteria for
handoff to an available analog radio channel and when the
estimated time dispersion corresponds to the criteria for
handoff, transmitting to the mobile station information on
the available analog radio channel and performing handoff of
the connection to the available analog radio channel.
8. A method according to claim 7, comprising the further
steps of:
estimating time dispersion of a possible digital radio
channel available to the mobile station during the time the
connection continues on the analog radio channel; and
comparing the estimated time dispersion of the possible
digital radio channel with criteria for handoff to the
possible digital radio channel and when the estimated time
dispersion corresponds to the criteria for handoff to the


33
possible digital radio channel, transmitting to the mobile
station information on the possible digital radio channel and
performing handoff of the connection to the possible digital
radio channel.
9. The method of claim 8 wherein the digital radio channel,
the analog radio channels and the possible digital radio
channel are of different frequencies.
10. The method of claim 7 wherein the digital radio channel
and the analog radio channels are of different frequencies.
11. In a multi-channel mobile radio system comprising a base
station and a mobile station designed both for digital
transmission of information using digitally modulated radio
signals on a digital radio channel and for analog
transmission of information using analog-modulated radio
signals on an analog radio channel, which mobile station
comprises an equalizer designed for a particular maximum time
dispersion of digitally modulated radio signals, a method of
maintaining a connection comprising the steps of:
estimating radio propagation properties of the digital radio
channel used by the mobile radio station for an ongoing
connection; and
comparing the estimated radio propagation properties with
criteria for handoff from the digital radio channel and when
the estimated radio propagation properties correspond to the
criteria for handoff, transmitting to the mobile station
information on an available analog radio channel and


34
performing handoff of the connection to the available analog
radio channel.
12. A method according to claim 11, comprising the further
steps of:
estimating radio propagation properties of a possible digital
radio channel available to the mobile station during the time
the connection continues on the analog radio channel; and
comparing the estimated radio propagation properties of the
possible digital radio channel with criteria for handoff to
the possible digital radio channel and when the estimated
radio propagation properties of the possible digital radio
channel correspond to the criteria for handoff to the
possible digital radio channel, transmitting to the mobile
station information on the possible digital radio channel and
performing handoff of the connection to the possible digital
radio channel.
13. The method of claim 12 wherein the digital radio channel,
the analog radio channels and the possible digital radio
channel are of different frequencies.
14. The method of claim 11 wherein the digital radio channel
and the analog radio channels are of different frequencies.
15. In a multi-channel mobile radio system comprising a base
station and a mobile station designed both for digital
transmission of information using digitally modulated radio
signals on a digital radio channel and for analog




35

transmission of information using analog-modulated radio
signals on an analog radio channel, which mobile station
comprises an equalizer designed for a particular maximum time
dispersion of digitally modulated radio signals, a method of
maintaining a connection comprising the steps of:
estimating radio propagation properties of a first digital
radio channel used by the mobile radio station for an ongoing
connection;
estimating radio propagation properties of a second digital
radio channel available to the mobile station;
comparing the estimated radio propagation properties of the
first and second digital radio channels and when the
estimated radio propagation properties of the first and
second digital radio channels correspond to criteria for
handoff to the second digital radio channel, transmitting to
the mobile station information on the second digital radio
channel and performing handoff of the connection to the
second digital radio channel; and
when the estimated radio propagation properties of the first
and second digital radio channels do not correspond to the
criteria for handoff to the second digital radio channel,
estimating the radio propagation properties of an analog
radio channel available to the mobile radio station and
comparing the estimated radio propagation properties of the
first digital radio channel and the analog radio channel with
criteria for handoff to the analog radio channel, and when
the estimated radio propagation properties of the first




36

digital radio channel and the analog radio channel correspond
to criteria for handoff to the analog radio channel,
transmitting to the mobile station information on the analog
radio channel and performing handoff of the connection to the
analog radio channel.

16. A method according to claim 15, comprising the further
steps of:
estimating the radio propagation properties of a possible
digital radio channel available to the mobile station during
the time the connection continues on the analog radio
channel; and
comparing the estimated radio propagation properties of the
possible digital radio channel with criteria for handoff to
the possible digital radio channel and when the estimated
radio propagation properties of the possible digital radio
channel correspond to the criteria for handoff to the
possible digital radio channel, transmitting to the mobile
station information on the possible digital radio channel and
performing handoff of the connection to the possible digital
radio channel.

17. The method of claim 16 wherein the first digital radio
channel, the second digital radio channel, the analog radio
channels and the possible digital radio channel are of
different frequencies.

18. The method of claim 16 wherein the first digital radio
channel, the second digital radio channel and the analog



37

radio channels are of different frequencies.

19. In a multiple channel mobile radio system comprising a
base station and a mobile station designed both for digital
transmission of information using digitally modulated radio
signals on any of a plurality of digital radio channels and
for analog transmission of information using analog-modulated
radio signals on any of a plurality of analog radio channels,
which mobile station comprises an equalizer designed for a
particular maximum time dispersion of digitally modulated
radio signals, a method of maintaining a connection
comprising the steps of:
estimating a signal level and bit error rate of a first
digital radio channel used by the mobile radio station for an
ongoing connection;
estimating radio propagation properties of a second digital
radio channel available to the mobile station;
comparing the estimated signal level and bit error rate of
the first digital radio channel and the estimated radio
propagation properties of the second digital radio channel
and, when the estimated signal level and bit error rate of
the first digital radio channel and the estimated radio
propagation properties of the second digital radio channel
correspond to criteria for handoff to the second digital
radio channel, transmitting to the mobile station information
on the second digital radio channel and performing handoff of
the connection to the second digital radio channel; and




38

when the estimated signal level and bit error rate of the
first digital radio channel and the estimated radio
propagation properties of the second digital radio channel do
not correspond to the criteria for handoff to the second
digital radio channel, estimating the radio propagation
properties of an analog radio channel available to the mobile
radio station and comparing the estimated radio propagation
properties of the first digital radio channel and the analog
radio channel with criteria for handoff to the analog radio
channel and when the estimated radio propagation properties
of the first digital radio channel and the analog radio
channel correspond to criteria for handoff to the analog
radio channel, transmitting to the mobile station information
on the analog radio channel and performing handoff of the
connection to the analog radio channel.

20. A method according to claim 19, comprising the further
steps of:
estimating radio propagation properties of a possible digital
radio channel available to the mobile station during the time
the connection continues on the analog radio channel;
comparing the estimated radio propagation properties of the
possible digital radio channel with criteria for handoff to
the possible digital radio channel and when the estimated
radio propagation properties of the possible digital radio
channel correspond to the criteria for handoff to the
possible digital radio channel, transmitting to the mobile
station information on the possible digital radio channel and
performing handoff of the connection to the possible digital




39

radio channel.

21. The method of claim 20 wherein the first digital radio
channel, the second digital radio channel, the analog radio
channel and the possible digital radio channel are of
different frequencies.

22. The method of claim 19 wherein the first digital radio
channel, the second digital radio channel and the analog
radio channel are of different frequencies.

23. In a mobile radio system having a mobile station and a
base station having use of analog radio channels and digital
radio channels subject to time dispersion whereby signal
intelligence is smeared over a wider time interval during the
course of transmission, a method of maintaining a connection
established on a digital radio channel, comprising the steps
of:
determining when time dispersion on said digital radio
channel exceeds a predetermined maximum acceptable time
dispersion; and
transmitting to the mobile station, when the time dispersion
of said digital radio channel exceeds the predetermined
maximum acceptable time dispersion, information on an
available analog channel, and performing handoff of the
connection to the available analog channel.

24. The method of claim 23 wherein said determining step
comprises estimating said time dispersion of said digital



40

radio channel at said mobile station and reporting estimates
to said base station.

25. The method of claim 23 wherein said digital radio channel
is an uplink channel and said determining step comprises the
base station estimating said time dispersion of said digital
radio channel.

26. The method of claim 23 wherein the digital radio channel
and the analog radio channel are of different frequencies.

27. In a multi-channel mobile radio system comprising a base
station and a mobile station designed both for digital
transmission of information using digitally modulated radio
signals on a digital radio channel and for analog
transmission of information using analog modulated radio
signals on an analog radio channel, which mobile station
comprises an equalizer designed for a particular maximum time
dispersion of digitally modulated radio signals, a method of
maintaining a connection started on the digital radio channel
comprising the steps of:
estimating time dispersion of the digital radio channel used
by the mobile radio station for the connection;
estimating radio propagation of an analog radio channel
available to the mobile station; and
comparing the estimated time dispersion and radio propagation
properties with criteria for handoff to the available analog
radio channel and when the estimated time dispersion and



41

radio propagation properties correspond to the criteria for
handoff, transmitting to the mobile station information on
the available analog radio channel and performing handoff of
the connection to the available analog radio channel.

28. The method of claim 27 wherein the digital radio channel
and the analog radio channels are of different frequencies.

29. A method according to claim 27, comprising the further
steps of:
estimating time dispersion of a possible digital radio
channel available to the mobile station during the time the
connection continues on the analog radio channel; and
comparing the estimated time dispersion of the possible
digital radio channel with criteria for handoff to the
possible digital radio channel and when the estimated time
dispersion of the possible digital radio channel
substantially corresponds to the criteria for handoff to the
possible digital radio channel, transmitting to the mobile
station information on the possible digital radio channel and
performing handoff of the connection to the possible digital
radio channel.

30. The method of claim 29 wherein the digital radio channel,
the analog radio channels and the possible digital radio
channel are of different frequencies.

31. In a multi-channel mobile radio system comprising a base
station and a mobile station designed both for digital



42

transmission of information using digitally modulated radio
signals on a digital radio channel and for analog
transmission of information using analog-modulated radio
signals on an analog radio channel, which mobile station
comprises an equalizer designed for a particular maximum time
dispersion of digitally modulated radio signals, a method of
maintaining a connection comprising the steps of:
estimating radio propagation properties of the digital radio
channel used by the mobile radio station for an ongoing
connection;
estimating radio propagation properties of an analog radio
channel available to the mobile radio station; and
comparing the estimated radio propagation properties of the
digital radio channel and the available analog radio channel
with criteria for handoff from the digital radio channel to
the available analog radio channel and when the estimated
radio propagation properties of the digital radio channel and
the available analog radio channel correspond to the criteria
for handoff, transmitting to the mobile station information
on the available analog radio channel and performing handoff
of the connection to the available analog radio channel.

32. The method of claim 31 wherein the digital radio channel
and the analog radio channels are of different frequencies.

33. A method according to claim 32, comprising the further
steps of:
estimating radio propagation properties of a possible digital




43

radio channel available to the mobile station during the time
the connection continues on the analog radio channel; and
comparing the estimated radio propagation properties of the
possible digital radio channel with criteria for handoff to
the possible digital radio channel and when the estimated
radio propagation properties of the possible digital radio
channel correspond to the criteria for handoff to the
possible digital radio channel, transmitting to the mobile
station information on the possible digital radio channel and
performing handoff of the connection to the possible digital
radio channel.

34. The method of claim 33 wherein the digital radio channel,
the analog radio channels and the possible digital radio
channel are of different frequencies.

35. In a multi-channel mobile radio system comprising a base
station and a mobile station designed both for digital
transmission of information using digitally modulated radio
signals on a digital radio channel and for analog
transmission of information using analog-modulated radio
signals on an analog radio channel, which mobile station
comprises an equalizer designed for a particular maximum time
dispersion of digitally modulated radio signals, a method of
maintaining a connection comprising the steps of:
estimating ratio propagation properties of a first digital
radio channel used by the mobile radio station for an ongoing
connection;


44

estimating radio propagation properties of a second digital
radio channel available to the mobile station;
comparing the estimated radio propagation properties of the
first and second digital radio channels and when the
estimated radio propagation properties of the first and
second digital radio channels correspond to criteria for
handoff to the second digital radio channel, transmitting to
the mobile station information on the second digital radio
channel and performing handoff of the connection to the
second digital radio channel; and
when the estimated radio propagation properties of the first
and second digital radio channels do not correspond to the
criteria for handoff to the second digital radio channel,
transmitting to the mobile station information on an
available analog radio channel and performing handoff of the
connection to the analog radio channel.

36. The method of claim 35 wherein the first digital radio
channel, the second digital radio channel and the analog
radio channels are of different frequencies.

37. In a multi-channel mobile radio system comprising a base
station and a mobile station designed both for digital
transmission of information using digitally modulated radio
signals on any of a plurality of digital radio channels and
for analog transmission of information using analog-modulated
radio signals on any of a plurality of analog radio channels,
which mobile station comprises an equalizer designed for a
particular maximum time dispersion of digitally modulated
radio signals, a method of maintaining a connection




45

comprising the steps of:
estimating a signal level and bit error rate of a first
digital radio channel used by the mobile radio station for an
ongoing connection;
estimating the radio propagation properties of a second
digital radio channel available to the mobile station;
comparing the estimated signal level and bit error rate of
the first digital radio channel and the estimated radio
propagation properties of the second digital radio channel
and when the estimated signal level and bit error rate of the
first digital radio channel and the estimated radio
propagation properties of the second digital radio channel
correspond to criteria for handoff to the second digital
radio channel, transmitting to the mobile station information
on the second digital radio channel and performing handoff of
the connection to the second digital radio channel; and
when the estimated signal level and bit error rate of the
first digital radio channel and the estimated radio
propagation properties of the second digital radio channel do
not correspond to the criteria for handoff to the second
digital radio channel, transmitting to the mobile station
information on an available analog radio channel and
performing handoff of the connection to the analog radio
channel.

38. The method of claim 37 wherein the first digital radio
channel, the second digital radio channel and the analog
radio channel are of different frequencies.

Description

Note: Descriptions are shown in the official language in which they were submitted.





1
METHOD OF MATNTATNTNG AN ESTABLTSHED CONNECTTON TN A MOBTLE RADTO
S°1STEM COMPRTSTNG BOTH ANALOG AND DTGTTAL RADTO CHANNELS
FIELD OF INVEN'fION
The present invention relates to the technical field of mobile
radio systems. More specificly the invention relates to a method
in a mobile radio system having both analog and digital radio
channels. The purpose of the invention is that a connection which
has begun on a digital radio channel often shall be possible to
sustain even when the dispersions on available digital radio
channels exceed the maximum time dispersion for which the equa-
lizers in the mobile stations have been designed. Due to its
character the method might be referred to the technical field of
handover in mobile radio systems.
BACKGROUND OF THE INVENTION .
25 The mobile radio systems first in common use had analog trans-
mission of information between base stations and mobile stations,
i.e. analog information was transmitted between base and mobile
stations by using analogously-modulated radio signals on analog
radio channels. By frequency multiplexing technique analog mobile
radio systems may have plural radio channels. The Nordic mobile
telephone system, NMT, is an example of such an analog mobile radio
system leaving many radio channels.
Recently there has been suggested mobile radio systems with
digital transmission of information between base stations and
mobile stations, i.e. digital or digitized information is trans-
mitted between base and mobile stations using digitally modulated
radio signals on digital radio channels. Of cause more radio
channels may be created by frequency multiplex in digital mobile
radio systems also. In order to get even mor<~ channels in digital
mobile radio systems it has been proposed that digital radio
charnels shall share a radio frequency in time multiplex. 'fhe pan
european mobile radio system, GSM, may be mentioned as an example
of SLlCh a digital mobile radio system.




2
The digital 'technique means substantial advantages in mobile radio
systems. Accordingly there is an interest in introducing the digi-
tal technique. On the other hand there are in certain areas exis-
ting analog mobile radio systems using the frequency band
available for a digital system. When huge investments have been
made in existing analog systems it is not always reasonable to
immediately replace an existing analog system with a digital
system. It has therefore been suggested to successively replace
analog channels in analog systems having many analog channels in
frequency multiplex by a number of digital channels in time
multiplex. For USA it has been suggested that each analog channel
shall be replaced by three digital channels in time multiplex.
During a long transitory period of time such a mobile radio system
would then comprise both analog radio channels for analog
transmission of information by using analog-modulated radio
signals and digital radio channels for digital transmission of
information by using digitally modulated radio signals. Mobile
stations which during the transitory time wish to be able to use
the full capacity of such a system must then be able to use both
analog radio channels and digital radio channels.
In digital mobile radio systems problems occur with time disper-
sion on the digital radio channels due to reflexions and multipath
propagation unless particular measures are taken. The time disper-
sion changes from one place to the other due to the radio propaga-
lion properties. In order to obtain a reliable transmission of
information in spite of time dispersion it is well known to have
equalizers at the receiving side in a mobile radio system.
Depending upon design such an equalizer can handle shorter or
longer time dispersion. One way to descibe the ability of an
3U equalizer to cope with time dispersion is to state the maximum
time dispersion in microseconds the eqwualizer is designed .for.
Another way is to state how many symbol time spans the equalizer
can cope with. The comp7.exity, cost and power consumption of an
equalizer increases progressively with the maximumtime dispersion
in microseconds and the number of symbol duration times. Accor-
dingly there is an interest in designing a mobile radio system
such that the requirements on the equalizers do not become to



3
great and in particular not become greater than necessary. or 'the
above mentioned pan eurapean system GSM it has been deceided that
the equalizers shall be designed to cope with a maximum 'time dis-
persion of 16 microseconds, which corresponds to four symbol time
intervals. In the above mentioned mobile radio system for USA
having both digital and analog radio channels the system should
probably need to cope with a maximum 'time dispersion of about 60
microseconds on the digital radio channels, which is more than the
duration of one symbol but less 'than the duration of two symbols.
The analog transmission of information on the analog radio
channels of existing analog mobile radio systems, e.g. the above
mentioned NMT or of the type existing in the USA, is normally
substantially less sensitive than digital systems to reflexions
and multipath propagation causing dispersion on the digital radio
channels.
BRIEF DESCRIPTION OF THE INVENTION
The complexity, cost and current consumtion of equalizers increase
progressively with the maximum time dispersion that an equalizer
shall be able to handle or the number of symbol duration times the
equalizer shall handle. This causes problems when designing mobile
radio systems where a long time dispersion must be considered: The
problems become particulary great when for various reasons i~t is
not possible to choose certain parameters mare or less freely,
e.g. when desiring to successively change from ana:tog technique
to digital technique in a mobile radio system by successively
replacing analog radio channels by time multiplexed digital radio
channels. It can become particularly unfavourable if the time
dispersion the system ought to successfully handle is substantial-
ly greater than a certain integer number of symbol time intervals
but at the same 'time substantially less than the closest succee-
ding higher integer number of symbol time intervals. If then
desiring to cope with the entire time dispersion by an Viterbi
equalizer it is necessary to design 'the equalizer for the biggest
one of the two integer numbers of symbol duration, ~.Che invention




4
'~ c~ :~ ;~ ,~
aims at solving this problem in a mobile radio system having both
analog and digital radio channels.
Somewhat simplified one could say that the invention is based upon
the idea to use the lower sensity of the analog radio channels for
reflexions and multipath propagation causing dispersion on the
digital radio channels. The invention is also based on the
understanding 'that the time dispersion on a particular radio
channel in most cases is substantially less than the maximum time
dispersion the system is to be designed for. Somewhat simplified
it could the be said that according to the invention the equa-
lizers are designed for a lower maximum time dispersion corre-
sponding to the lower of the integer numbers of symbol time inter-
vals and instead at least a temporary change over from a digital
radio channel to an analog radio channel is done when the time
dispersion on the used digital radio channel becomes greater than
the maximum time dispersion the equalizer is designed for. Accor-
dingly handover is done to an available analog radio channel. If
there should be another available digital radio channel being
sufficiently goad the first choice is to attemt to use this by
handover to such a digital radio channel. In such cases handover
to an available analog radio channel is done only when there is
no sufficiently good available digital radio channel.
It is possible but not always self evident to return to a digital
radio channel by handover as soon as there is an available digital
radio channel being sufficiently good.
Tn order to get to know in time when the dispersion on the used
digital radio channel tends to become too severe the radio propa-
gation properties of the used digita:L radio channel ought to be
estimated more or less continousl.y. The estimation is preferably
done by estimation of signal level and symbol error .frequency,
which can be performed in ways known per see. One method has been
presented by Italtol within the framework on the pan european GSM
system and is disclosed in a GSM paper titled GSM/WP2 doc 1.7/88.
Based on the method suggested by Italtel Ericsson has suggested a
method of measuring and transferring bit error rate information




5
for the future U.S. digital cullular standard in a paper. forwarded
to the TIA Technical Subcommittee, TR°45 Digital Cellular
Standards, meeting an August 29, 1989 in San Diego, California.
The estimation of signal level and symbol error frequency is
preferably performed at the mobile station which reports the
estimation to its base station. This is in accordance both with
the GSM standard and the TIA on digital cellular mobile radio
systems.
Instead of estmating time dispersion indirectly by estimation of
bit error rats and signal level the time dispersion may be esti°
mated directly using receiver equalizer and synchronizing words
transmitted on digital radio channels. Such direct estimation may
use methods based on the contribution to the 38th TEEE vehicular
technology conference held in Philadelphia, Pennsylvania, USA on
15°1? June, 1988, titled "Bit synchronization and timing sensi-
tivity in adaptive viterbi equalizers for narrowband°TDMA digital
mobile radio systems", by A. Baier, G. Heinrich, U. Wellens.
When possible it is preferred to perform a more or less continous
estimation also for at least on other digital radio channel than
the channel presently used by the mobile. At least 'the radio
propagation properties of at least one other possible digital
radio channel shall be performed when the estimation of the radio
propagation properties of the used digital radio channel indicates
that the used digital radio channel is or tends to be too poor.
In order to increase the likelyhood of successful handaver to an
analog radio channel the properties of the intended analog radio
channel also ought to be estimated in an appraprate way, e.g. the
expected signal level and/or the signal to noise ratio of the
channel may be estimated. When 'this is difficult to accomplish at
the mobiles :it may be done at the base stations.
When introducing digital radio channels into a known mobile radio
system with analog radio channels where decisions on handoff are
done in the fixed part of the system and estimation of dispersion


CA 02030607 2000-10-03
6
and other radio signal properties may also be done in the
fixed part of the system the normal known handoff signalling
procedure of the system for analog radio channels may be
used. In a mobile radio system with a mobile assisted
handoff procedure where at least some measurements or
estimations are done at the mobile the procedure for handoff
from a digital radio channel to an analog radio channel due
to dispersion must include some signalling of results of
measurements or estimations from mobile to base station.
Therefore, in accordance with the invention there is
provided in a mufti-channel mobile radio system comprising
base stations and mobile stations designed both for digital
transmission of information using digitally modulated radio
signals on a digital radio channel and for analog
transmission of information using analog-modulated radio
signals on an analog radio channel, which mobile stations
comprise equalizers designed for a particular maximum time
dispersion of digitally modulated radio signals, a method of
maintaining a connection using a digital radio channel
comprising the steps of: estimating the time dispersion of
at least the digital radio channel used by a mobile radio
station for the connection; and when the time dispersion of
the digital radio channel is estimated to be too big,
performing handover to an analog radio channel of the system
assumed to have sufficiently good radio propagation
properties if such an analog radio channel is available to
the mobile station.


CA 02030607 2000-10-03
6a
THE DRAWINGS
Figure 1 illustrates part of a cellular mobile radio system
with cells, a mobile switching centre, base stations and
mobile stations.
Figure 2 illustrates a plurality of radio channels within a
frequency band and use of some radio channels in a cellular
mobile radio system according to figure 1.
Figure 3 illustrates use of radio channels according to
figure 2 for control channels, analog communication channels
and time division multiplex digital communication channels in
a cellular mobile radio system according to figure 1.
Figure 4 illustrates bursts separated by guard spaces on a
radio channel used for digital communication channels in time
division multiplex according to figure 3 in a cellular mobile
radio system according to figure 1.
Figure 5 illustrates a base station in a cellular mobile
radio system according to figure 1 with radio channels used
according to figures 2 to 4.
Figure 6 illustrates a mobile station in a cellular mobile
radio system according to figure 1 for communication with a
base station according to figure 5 on control and digital
communication channels according to figures 2 to 4.




Figure 7 illustrates actual error rate and estimation of error
rate on a digital channel by using additional channel coder at
the receiving side of a base or mobile station.
Figure 8 illustrates impulse response of digital channel radio
link including transmission and reveiving means and estimation of
time dispersion on a digital channel using synchronization words.
DETAILED DESCRTPTION OF EMBODIMENTS
Figure 1 illustrates ten cells C1 to C10 in a cellular mobile radio
system. Normally a method according to the invention is imple-
mented in a cellular mobile radio system comprising many more
cells than ten. However, for the purpose of eplaining the inven-
tion ten cells may be sufficient.
For each cell C1 to C10 there is a base station B1 to B10 with the
same number as the cell. Figure 1 illustrates base stations in the
vicinity of cell centre and living omnidirectional antennas. The
base stations of adjacent cells may however be cvlocated in the
vicinity of cell borders and have directional antennas as is well
known to those skilled in the art.
Figure 1 also illustrates ten mobile stations M1 to M10 within a
cell and from one cell tv another cell. Normally a method accor-
ding to the invention is implemented in a cellular mobile radio
systems comprising many more mobile stations than ten. In par-
ticular the mobile stations are normally times as many as the base
station. However for the purpose of explaining the invention ten
mobile stations may be sufficient.
Also illustrated in Figure 1 is a mobile switching centre MSC. The
mobile switching centre illustrated in Figure 7. is connected 'to
all ten illustrated base stations by cables, The mobile switching
centre is connected by cables also to a fixed public switching
telephone network or similar network with ISDN facilities. All
cables .farm the mobile switching centre to base stations and
cables to the fixed network are not illustrated.




8
In addition to 'the mobile switching centre illustrated there may
be another mobile switching centre connected by cables to other
base stations than those illustated in Figure 2. Instead of cables
other means may be used for base to mobile switching center
communication, e.g. fixed radio links.
The cellular mobile radio system illustrated in Figure 1 comprises
a plurality of radio channels for communication. The system is
designated both for analogue information, e.g. speech, digitized
analogue information, e.g. digitized speech, and pure digital
information, e.g. pure data. In this application the term
connection is used for a communication channel between a mobile
station and another mobile station in the same system or another
system or a fixed telephone or terminal in a fixed network
connected to the cellular mobile radio system. Thus a connection
may be a call where twa persons talk to each other but may also be
a data communication channel where computers exchange data.
Figure 2 illustrates somewhat simplified a plurality of radio
channels RCH1 to RCH2n withing a frequence band. A first group of
radio channels RCH1 to RCHN are used in the cellular mobile radio
system for transmission of radix signals from base stations to
mobile stations. A second group of radio channels RCHN+1 to RGHD12
are used in the cellular mobile radio system for transmission of
radio signals from mobile stations to base stations.
Some of the radio channels are used for control channels. Normally
each base station has at least one control channel. Normally a
cantrol channel is not used for transfer of information on a
connection but for monitoring and control of mobiles during setup
of a connection, maintaining as established connection and handoff
of an established connection. In Figure 3 is illustrated how the
radio channel RG~If all the time is used for a control channe:L CCHIc
while the radio channel RCHg all the time is used for a control
channel CCHr.
Some of the radio channels are used for analogue communicatian
channels. Normally analogue communication are used for connections




9
CD Y
where analogue informations is exchanged, e.g. telephone ells
where two persons talk to each other. Normally one analogue
communication channel is required for each such connection. When
a radio channel is used for an analogue communication channel the
information on the connection is transmitted with analogue
modulated radio signals. In addition to the information on the
connection the analogue communication channel may also be used for
associated information, e.g. a supervisory audio tone. In figure
3 is illustrated how the radio channel RCHa all the time is used
for an analogue communication channel ACHi while the radio channel
RCHb all the time is used for an analogue communication channel
ACHv. Normally each base station has at least one radio channel
used for analogue communication channel.
Some of 'the radio channels are used for digital communication
channels. Normally digital communication channels are used far
connections where digital or digitized information is exchanged,
e.g. data or digitized speech. The radio channels used for digital
communication channels are divided into time slots and the 'time
slots are grouped in frames. The time slots are allotted to
digital communication channels whereby multiple digital channels
share a common radio channel in time division multiplex. In Figure
3 is illustrated a radio channel RCHc having three time slots in
each frame F. A first of the time slots is allotted to the digital
communisation channel DCH4, a second of the time slots is allotted
to the digital communication DCHS AND the third of the time slots
is allotted to the digital communication channel DCH6. Thus the
radio channel RCHc is used for three digital communication
channels. Figure 3 also illustrates how the radio channel RCHd is
used for three digital communication channels DCFI7, DCH8 and DCH9
3 0 in a corresponding way .
Tn figure 3 the frames F of radio channels RCHc and RCHd have three
'time slots. Depending on the required bandwidth of the various
digital communication channels it is conceivable to have less
slots in a frame, e.g. two slots, or to have more slots in frame,
e.g. six slots. When the digital communication channels are used
for connections where digitized speech is exchanged, six time




10
N
slots may give too poor speech quality when the radio bandwidth is
3Q kHz.
Un a radio channel used for digital communcation channels the base
or mobile station transmits a time slot identifier code with the
radio signals at least in every time slot used for a connection.
On a particular radio channel, e.g. RCHc, the time slot identifier
codes in different time slots are different. thus 'the time slots
identifier node T11 is transmitted in the first time slot of radio
channel RCHc allotted to digital communication channel DCH4. The
time slot identifier code T12 is transmitted in the second time
slot of radio channel RCHc allotted to digital communication
channel DCH5. The time slot identifier code T13 is transmitted in
the third time slot assigned to digital communication channel
DCH6.
The same time slot identifier codes may be used on two or more
radio channels, possibly all radio channels. Figure 3 illustrates
how the time slot identifier T11 is transmitted inthe first 'time
slot of radio channel RCHd allotted to digital communication
channel DCH7. The time slot identifier cods T12 is transmitted in
the second time slot of radio channel RCHd allotted to digital
communication channel DCH8. The time slat identifier code T1,3 is
transmitted in the third time slot of radio channel RCHd allotted
to digital communication channel DCH9. Thus the time slot
identifier code does not alone identify the chancel but identifies
the time slot in a frame. It is conceivable to have one set of time
slot identifiers T11 to T13 fox channels with three slot frames F
and have a different set of time slot identifiers T14 to T9 for six
slot frames whereby the time slot identifier also may indicate the
number of slots in a frame of the radio channel.
On a radio channel used for digital radio channels also trans-
mitted a digital voice colour code with the radio signals at least
in each time slot used for a connection. On a particular radio
channel the same digital voice colour code is transmitted with
the radio signals in different time slots. figure 3 illustrates
the transmission of the same digital voice colour node VC1 in all




11
time slots of 'the radio channel RCHc. Normally the same digital
voice colour code is used for all radio channels to and .from a
particular base station, e.g. the digital voice colour code VC1 is
used for all radio channels to and from base station BS1.
Some adjacent base stations may use the same digital voice colour
code e.g. base stations B2, B6 and B7 may use the same digital
voice colour code as long as base station BS1. Other base stations
use a different digital voice colour code, e.g. base stations B4,
B5, B9 and B10 all use the digital colour code VC4. Still other
base stations use another different digital voice colour code,
e.g. base stations B3 and B8 use the digital voice colour code
VC7.
The radio signals on a radio channel used for 'time division
multiplex digital communication channels are transmitted in
bursts. Figure 4 illustrates a burst in a time slot separated by
guard sFaces from the end of the preceeding burst and the begin-
ning of a succeeding burst in adjacent time slots.
A transmitted burst comprises at least a time slot identifier,
abbreviated T1, and a digital voice colour code, abbreviated VC,
and normally also information to be forwarded to the other party
of the connection as well as connection or channel associated
information for control or monitoring purposes.
:It is well known to those skilled in the art that 'there is a need
for receiver synchronisation in time division multiple access
radio communication systems, For this purpose it is well known to
transmit synchronisation words or patterns in each burst and to
transmit particular frame synchronisation words or patterns from
a master or base station to a slave or mobile station. Preferably
the time slot identifier code according to the present invention
may als-o be used fo.r synchronisation of receiver 'totransmitter.
Selecting time slot identifier codes in this way means that the
implementation of time slot identifier codes according to the
present invention does not require any dedicated bits for the 'time
slot identifier. Teorectically there exist a number of uncor.re--




12
laced binary mufti bit words, e.g. 26 bit words, which may be used
as different synchronisation words. According to the present
invention one unique time slot identifier and synchronisation
words is required for each time slot in a frame on the channel. For
the purpose of both time slat identification and synchronisation
the time slot identifier code words are to be chasm for minimum
correlation between each other and between themselves except far
correlation to themselves in phase. Thus a first time slot
identifier code used to identify the first time slat of a frame
according to the present invention and to synchronise receiver to
transmitter shall exhibit low correlation to an identical code
when substantially out of phase but exhibit high correlation to an
identical code when substantially in phase. A second time slot
identifier code used to identify 'the second time slot of a frame
and 'to synchronise receiver to transmitter shall also be exhibit
low correlation to an identical code when out of phase but exhibit
high correlation to an identical code when substantially in phase.
All other time slot identifiers used on the channel should also
exhibit low correlation to an identical code when out of phase but
exhibit high correlation to an identical code when in phase. Any
time slot identifier node used an the channel should also exhibit
low correlation to any other time slot identifier code used on the
channel irrespective of phase relationship, i.e. of in phase or
out of phase. Once the requirements on time slot identifier codes
axe given one skilled in the art will be able to select proper time
identifier codes withawt inventive activity. However, far the
convenience of those riot skilled in 'the art the following eight 26
bit words are given as examples of possible time slot identifier
codes for an eight slat frame:
code for time slot 1:
(0,0,1,0,0,1,0,1,1,1,0,0,0,0,1,0,0,0,1,0,0,1,0,1,1,1,)
code for 'time slot 2:
(0,0,1,U,1,1,0,1,1,1,0,1,1,1,1,0,0,0,1,0,1,1,0,1,1,1,,)
code fox' time slat 3:
(0,1,0,0,0,0,1,1,1,0,1,1,1,0,1,0,0,7.,0,0,0,0,1,1,1,0,)
code for t ime slot ~
(0,1,0,0,0,1,1,1,1,0,1,1,0,1,0,0,0,1,0,0,0,1,1,1,1,0,)
code for time slot 5:




13
(0,0,0,1,1,0,1,0,1,1,1,0,0,1,0,0,0,0,0,1,1,0,1,0,1,1,)
code .for time slot &:
(0,1,0,0,1,1,1,0,1,0,1,1,0,0,0,0,0,1,0,0,1,1,1,0,1,0,)
code for time slot 7:
(1,0,1,0,0,1,1,1,1,1,0,1,1,0,0,0,1,0,1,0,0,1,1,1,1,1,)
code for time slot 8:
(1,1,1,0,1,1,1,1,0,0,0,1,0,0,1,0,1,1,1,0,1,1,1,1,0,0,)
When there are less than eight time slots in a frame on the radio
channel less than all of the given code words may be used. However
there may be advantages in using other than the given code when
there are only three time slots in a frame.
It of course conceivable to use binary time slot identifier codes
having more or less than 26 bits. Longer code words offer some
advantages but has the disadvantage of occupying more of the
available space in a burst.
In the advanced mobile phone service system, abbreviated A,.T~IPS,
there is a supvervisory audio tone, abbreviated SAT, transmitted
e.g. on analog communication channels. The reason for transmitting
the SAT in AMPS is that in an interference limited mobile radio
communication network there should be some means far the receiving
entity (e. g. base statian) to identify the transmitting entity
(e.g. mobile station) or at least with high likelyhood a~cclude
interchange of transmitter entity without the need for continous
transmission of transmitter identity. The object of the digital
channel code in a cellular mobile radio system according to the
present invention is at least partly the same as that of SAT in
AMPS. The number of different SATs is three in AMPS. The number of
different digital. voice colour codes in a cellular mobile radio
system according to the present invention is preferab:Ly much
higher than three, mainly to Support a proper introduction of
discontinous transmission. The number of different dzgi~tal colour
codes should be high enough to guarantee that, cells where the same
:radia channels are used having the same colour code ar at a
geographical distance from each other sufficient not to cause co-
channel interference. Tn case discontinous transmission is used




14 ~~~~~~'~.
the disturbing signal of an other station should not significantly
exceed the level of noise. For this purpose an eleven bit colour
code seems quite sufficient in normal cases. However, the longer
color codes the longer connection setup and handoff procedures.
Considering the possible load on control channels eight bit
digital voice codes seems to be a good compromise. Preferably the
digital voice channel code is binary word having eight bits,
enabling theoretically 256 different voice colour codes.
A separate digital voice colour code requires space in a burst
thus reducing the space available for speech or data on the
connection. However, 'the following implementation of voice code
does not .require any dedicated bits for the digital voice code in
a burst on a digital communication channel.
At the transmitting side the digital voice colour code is added
bit by bit modulo two to a field under error detection but without
error correction within the information part of the burst after
channel coding. On the receiving side the bursts in the time slots
are deinterleaved and checked. This check is done by adding bit by
bit the known digital voice colour code modulo two, as is done at
the transmitting side, before channel decoding and error detec-
tion. If no error is found after adding the digital voice colour
code modulo two the information part of the burst was sent from the
expected transmitter and not from an interferer.
Figure 5 illustrates a base station in a cellular mobile radio
system according to figure 1 with radio channels according to
' figures 2 to 4. The base station is designed for transmission and
reception on a plurality of radio channels used for digital
communication channels, analog communication channels anclcontrol
channels. In figure 5 not all base station equipment for all
channels is illustrated. Normally a base station has equipment for
more channels, in particular analogue communisation channels, as
well as other equipment for power supply, maintenance etc but the
illustrated equipment is believed sufficient for the understanding
of the present invention.
6




15
The base station is connected to a mobile switching centre by six
trunks. A first incoming trunk for digital communication channels
is connected to a digital trunk demultiplexor and interface DMU-
D. A second incoming trunk for analog communication channels is
connected to an analog trunk interface and demultiplexor DMU-A. A
third incoming trunk for control channels and bass station control
information is connected to a trunk interface and control
information demultiplexor DMU-C. A first outgoing trunk for
digital communication channels is connected to a digital multi-
plexor and trunk interface MUX-D. A second outgoing trunk for
analog communication channels is connected to an analog multi-
plexor and trunk interface MUX-A. A third outgoing trunk for
control channels and base station information is connected to a
control information multiplexor and trunk interface MUX-C.
For each of the outgoing radio channels used for digital com-
munication channels the base station comprises digital analog
channel coding means, DCC, connected to a digital trunk demul-
tiplexor and interface, DMU-D, modulo two adding means, M2A, burst
generation means, BG, modulator means, MOD, and power amplifica-
tion means, PA, connected to an antenna. Two such outgoing radio
channels share a digital voice colour code means, vCS, connected
to their modulo two adding means. Two such outgoing radio channels
also share a time slot identifier code means, TIS, connected to
their burst generators.
For each of the outgoing radio channels used for analog communica-
tion channels the base station comprises analog transmission
channel processing means, ATC, connected to an analocJ trunk
interface and demultiplexor, DMU-A, modulator means, MOD, and
power amp7.ification means, PA, connected to an antenna.
For each of the outgoing radio channel s used for control channels
the base station comprises outgoing control channel, processing
means, CTC, connected to the trunk interface and control informa-
tion demultiplexor, DMU-C, modulator means, MOD, and power
amplification means, PA, connected to an antenna.




2~~Q'
For each incoming radio channel used for digital communication
channels the base station comprises radio receiver means, REC,
connected to an antenna, radio signal strength or level measuring
means, SLM, analog to digital conversion means, A/D, multipath
equalizer and burst synchronizing and time slot recognition and
automatic frequency control means, EQ-AFC, modulo two adding
means, M2A and digital channel decoder means, DCD, connected to a
digital multiplexor and trunk interface, MUx-D.
Two incoming radio channels used for digital communication share
digital voice colour code means, VCS, connected to their modulo
two adding means. Two such incoming radio channels also share
digital channel bit error measuring means, BEM, connected to their
digital channel decoders, DCD.
For each incoming radio channel used for analog communication
channels the base station comprises radio receiver means, REC,
connected to an antenna, radio signal strength or level measuring
means, SLM, and incoming analog channel processing means, ARC,
connected to an analog multiplexor and trunk interface, MUX-A.
For each incoming radio channel used for control channel the base
station comprises radio receiver means, REC, connected to' an
antenna, radio signal strength or level measuring means, SLM, and
incoming control channel processing means, CRC, connected to the
control information multiplexor and trunk interface, MUX-C.
All modulation means and .radio receiver means are connected to
frequence synthesizer means, FS. The frequency synthesizer means
are controlled by a central processor, CP. The CP also controls
the DCCs, VCSs, BGs, EQ-AFCs, DCDs, BEM, ATC, ARC, CTC, CRC and
MUX-C. Preferably the central. processor is not the only processor
in the base ration but other means may also comprise processors,
in particu:Lar the ATC, ARC, CTC, CRC and EQ-AF'Cs.
The base station according to Figure 5 is untended for communica-
tuon with mobile statuon only having equipment for analog
communication channels and.control channels. The base station is




17
also intended f.or communication with mobile stations only having
equipment far digital communication channels alld comtrol channels.
The base station is also intended for communication with dual mode
mobile stations designed for communication on both analog anc~
digital communication channels as well as channels.
Mobile stations designed only for analog communication channels
may be of a kind well known to those skilled in the art and operate
according to AMPS standard. Futhermore, a method according to the
invention can only be implemented with mobile stations with means
for communication both on digital and on analogue radio channels.
Accordingly there is no need for disclosing a mobile designed only
for analogue radio channels or its operation here. Neither is
there any need for describing parts of a dual-mode mobile station
used only for communication on analogue radio channels and control
channels.
Figure 6 illustrates parts of a dual--mode mobile station in a
cellular radio system according to Figure 1 for communication with
a base station according to figure 5 on radio channels according
to figures 2 to 4. The parts illustrated are involved in com-
munication on digital radio channels. A dual mode mobile station
capable of using both analog and digital radio channels .thus
includes in addition to the means illustrated in figure 6 first
analog signal processing means connected 'to the microphone and the
modulator and second analog signal processing means connected to
the radio receiver IF stage and to the loudspeaker. ~.Che first and
second analog signal processing means are merely indicated by one
block and are controlled by the microprocessor.
The mobile station comprises a microphone connected to analogue to
digital speech coding means for coding speech or sound into a
binary code with a bitrate or bandwidth less than 11 k~Iz. prefera-
bly about 7 to 8 7c~Iz. Connected to the speech coding means is
channel coding means for interleaved error proctecting coding of
the digital information from the speech cader. The channel coder
is connected to a module two adder for modulo adding of a digital
voice colour code to the digital information from the channel




:L 8
~~~~a
coder. The modulo two adding means are connected to a burst
generator for co7.lecting information to be transmitted in a burst '
and arranging the information including time slot :indentifier code
into a proper burst. When the mobile is used for data transmission
on a connection or during connection setup the burst generator
puts data and/or control information in the burst instead of
digitized speech. Such information may be suplied from a keyboard
via a microprocessor and the channel coder or directly from the
microprocessor. A modulator is connected to the burst generator
for receiving digital information to be modulated one radio
frequency carrier from as frequency synthesizer arid be amplified
in a power amplifyer. The modulation method used may be continous
phase modulation or other types suitable for digital information
transmission. The power amplifyer is connected to an antenna via
a duplex and is controlled from the microprocessor.
The mobile station also comprises a radio receiver connected to
the duplexer, a radio signal or level means and analog to digital
conversion means. The radio receiver comprises RF and IF stages
with filter, demodulator etc. Means for equalizing the digital
communication channel and automatic frequency control and
automatic gain control are connected to the radio receiver and the
input of a modulo two adder. The modulo two adder adds a digital
voice colur code modulo two to the digital information from the
equalizer. The modulo two adder output is connected to a channel
decoder for deinterleaving and error detection and correction of
digital information from the modulo two adder. Means for conver-
ting digital information to analogue information or speech are
connected to the channel decoder and a loudspeaker.
When the mobile station is tuned to a radio channel used by a base
station for control channel some of the illustrated equipment of
the mobile station is not used, in particular not channel. and
speech decoder. When control and monitor9.ng information is
transmitted from the base station on the control channel according
to the AMPS standard the microprocessor receives and interpreter
s9.gnals from the analog 'to digital converter.




19
Except for the time slot identifier and the digital voice colour
code and means for introducing, recognizing and removing them from
the flow of information the mobile radio according to Figure 6 may
operate on time division multiplex digital communication channels
in a way similar to known digital mobile radio stations, e.g. of
the kind disclosed in the GSM standard or in Ericsson. Review No.
3, 1987.
Accordingly there is no need here to further describe in detail
the overall operation or the detailed operation of the various
means. Regarding the 'time slot identifier code and the digital
colour code the mobile station comprises means for storing all
possible time slot identifier codes and digital voice colour codes
to be used in communication with a base station. The micorproces-
sor receives instructions from 'the base as to which codes to use
in the particular connection and reads the codes from the stores
and supplies the modulo trrao adders and burst generator with i
appropriate time slot identifier code and digital voice colour
code.
When receiving radio signals from a base station synchronisation
and recognition of 'time slot identifier is performed in the
equalizer in cooperation with the microprocessor. Measurement of
bit errar rate on an established connection is performed in the
channel decodex in cooperation with the microprocessor. Equalizer]
and methods of synchronisation and bit error measurement are well
known to those skilled in the art. Accordingly there is no need
here to further describe such methods or means for performing
them. However for the person not skilled in the art a brief
explanation on bit error measurement for digital radio channels
will be given in connection with figure 6 and 7.
In this example it is assumed that at least a part of the digital
information transmitted an the digital radio channel is protected
by an error correction code. As an example, both the GSM and TIA
systems use 20 ms speech blocks with part of the speech codes
output bit: protected by a error cor section code . S ince the speech
codes operate block 'wise so does the error correction applied.




20
The actual bit error rate, BER, of the radio channel including
certain transmitting and receiving means at base and moblie
station is of course due to the difference between the information
actually leaving the channel coder of the transmitter and the
information received by the receiver decoder. This is illustrated
in the left hand part of. figure 7. An estimate of the actual bat
error rate can be done by reencoding the decoded data in the
receiver. and compare this bit stream with the input to the channel
decoder of the receiver. This is illustrated in a principal way in
the right hand part of figure 7. For the purpose of this an
additional channel encoder may be used and in figure 6 an additio-
nal encoder has been incorporated in the receiving part of the
mobile station. Corresponding additional encoder means may of
course be incorporated in the receiving part of a base station
according to figure 5. For comparing corresponding digital symbols
according to figure 7 the central pracessor in the base station
and the microprocessor of the mobile station may be used.
If the channel decader in the receiver has corrected all bit
errors in a block of n bits then the bit by bit comparison of the
received data with the reencoded data will be equal to the actual
number of bit errors in the block of n bits. The number of bit
errors is the number of diffarent bit values in the above men-
tioned comparison. Thus, in this case the estimated number of bit
errors is equal to the true value of bit errors. The bit errar rate
in the block of n bits is then the number of bit differences
divided by n.
If the channel decoder of the receiver is not capable of reprodu-
cing the transmitted n bits e.g. the channel decoder makes an
error, the estimated bit error rate will not be equal to the actual
3o error rate. Nevertheless, for bit error rates in the range of
interest the estimate will be high:Ly correlated to the actual
rate. The difference between the estimated and actual rate can be
seen as a measurement noise term. The system is untended to
operate with a radio link quality where 'most of the blocks can be
corrected by the channel decoder, otherwise the audio quality
would be unintelligible, thus the measurement noise is limited. If



21
the radio link quality is so low that almost every block is
uncorrectable by the channel decoder, the output from the decoder
is more or less a random pattern with low correspondence with the
received block. This can for instant happen if the amount of time
dispersion is much more than the equalizer can handle. The
important thing is that the bit error estimate will still cause a
generally correct conclusion, i.e. too low radio channel quality,
because the difference between two blocks with almost no corres-
pondence will result in that, in average, every second bit
comparison will indicate a bit error. Thus the bit error rate will
be estimated to be 50%.
In summary, independent of the radio link quality this method
described will indicate to the receiver the quality of the radio
link in terms of a bit error estimate.
The procedure for connection setup in a cellular mobile radio
system according to figure 1 with a base station according to
figure 5 and a mobile station according ~to figure 6 may be similar
to the corresponding procedure in AMPS when the channel to be used
is an analogue communication channel. However, when the channel to
be used for the connection is a digital communication channel
according to figures 3 and 4 the base station informs the mobile
station not only on radio channel but also on time slot and digital
voice colour code to be used. During the setup procedure the base
station then also informs the mobile station on a plurality of
radio channels the signal strength on which to be measured by 'the
mobile. Normally this plurality of radio channels are the radio
channels used for channels by adjacent bases/cells. Dpending upon
the movement of the mobiles as well as other circumstances a new
plurality of radio channels may be selected and corresponding
information be transmitted to the mobile from the responsible base
station during the course of the connection. During the course of
a connection for. which a digital communication channel is used the
mobile measures the signal strength of signals on 'the given
plurality of radio channels. Measurements may be done during time
slots not used by the digital communication channel. The mobile
station also measures signal strength on the digital communication




22
channel used for the established connection and the bit error rate
on the established connection. The mobile station transmits
results of its measurements, preferably averaged, frequently 'to
the base station, preferably twice a second.
In addition to or instead of measuring signal strength and bit
error rate the mobile station may estimate time dispersion on 'the
used digital radio channel using the time slot identifier code
words and the equalizer in a way to be described later. The mobile
station may transmit the estimation in addition to or instead of
signal level or bit error rate measurement results frequently to
the base station.
The base station also measures signal strength on the digital
communication channel used for established connection and the bit
error rate on the established connection. The base station may
also estimate time dispersion on incoming radio channels used for
digital cammunications channels. This may be done using the time
slot identifier cedes and equalizer in a way to be described
later. The base station processes and analyzes the results of its
own measurements and/or time dispersion estimates and the measure-
meat and/or time dispersion estimates of the mobile and compares
with criteria for handoff. When according to the results, and
criteria a handoff to another base station is desirous the base
station informs the mobile switching centre indicating at least
one target base station assumed suitable for 'taking over the
responsibility for the communication with the mobile.
The mobile switching centre request the 'target base station ( s) to
measure signal strength on a radio channel in a time slot used by
the mabile far the established connection. The mobile switching
centre also informs the target base station on the digital colour
code used by the mobile station.
The ~targe~t base stations) tune (s) a receiver to the radio
channel indicated by the mobile switcraing centre and uses the time
slot identifier of the indicated time slot for burst synchronisa-
tion. The 'target base station checks the appearance of 'the digital




23
colour code indicated byt 'the mobile switching centre and measures
the signal strength of the burst signals provided the digital
colour code is correct. The target base station then transmits its
results of signal strength measurement to the mobile switching
centre. The target base station also informs the mobile switching
centre on the result of the checking of the appearance of the
digital colour code, 'that is whether the digital voice colour code
appeared in the burst in the time slot of the radio channel.
The mobile switching centre determines whether handoff to a 'target
base should be performed taking the results of signal strength
measurements of target bases) into account as well as other
circumstances, e.g. traffic load.
When the mobile switching centre determines that handoff to an
other base station and digital radio channel shall be performed it
transmits to responsible base and target information an new radio
channel, new time slot and new voice colour code to be used by the
mobile station for the connection after handoff and new radio
channel to be used by target base station for the connection after
handoff.
The responsible base station forwards informationon about the two
new radio channels, new time slot and new digital colour code to
the mobile. After receiving this information the mobile station
tunes to the new radio channel to be used for the connection by the
target base station and looks for the new time slot identifier
code in received signals on the radio channel. The mobile station
uses the new time slot indentifier code in received signals for
burst synchronisation. After synchronisation and tuning its
transmitter to the new radio channel the mobile station begins
transmitting bursts in 'the new time slot on 'the new radio channel .
The new digital colour code is transmitted with each burst.
The target base station tunes a receiver to the new channel to be
used for the connection by the mobile station and looks for the
new time slot identifier code. The target base station uses the
time s7.at identifier code for synchronisation. The target base




2 ~l
station then looks for 'the new digital colour code in signals in
the new time slot of the new channel. If the target base station
identifies the new digital colour code in the bursts in the new
time slot of the new radio channel this is reported to the mobile
switching centre. The mobile switching centre then interpretes the
handoff as sucessful and acts accordingly. After sucessful handoff
the former target base station now being resposible base station
informs the particular mobile station on a new plurality of radio
channels the signal strength on which to be measured by the
mobile.
In the embodiment of the handoff method described above the
responsible base station and mobile station use the same time slot
identifier code and 'the same digital voice colour code. However,
it is conceivable to use different time slot identifier at base
and mobile stations for a particular connection.
In the embodiment of the handoff method described the mobile
station measures signal strength on radio channels used for
control by base station. However, it is conceivable to request
mobile station to measure signal strength on radio channels used
for digital communication channels by base station, in particular
when there are no radio channels entirely used for control
channels by base stations.
The procedure far intracell handoff from a digital radio channel
to an analog radio channel of the same base station is of course
less complex than the described handoff procedure because
responsible base station and target base station is the same. When
according to 'the results and criteria a handoff from a digital
radio channel to an analog radio channel is desirous the base
station informs the mobile switching centre. After approval by the
mobile switching centre the base station may perform the normal
procedure of the system for i.ntracell handoff to a new analog
radio channel, the procedure comprising transmitting informing to
'the mobile station on the new radio channel to be used. If the
invention is implemented in a mobile radio system where handoff
between analog channels is performed according to the AMPS




25
~~~~~b~~o'
standard the this procedure may also be used for handoff from
digital to analog handoff due to time dispersion. If the invention
is implemented in a mobile radio system where handoff between
analog channels is performed according to an other standard then
this procedure may also be used for handoff from digital to analog
channel due to time dispersion. Accordingly there is no need for
describing a complete such a procedure here. Suffice it to say
that there is no transmission of information on time slot iden-
tifier code or digital voice colour codes from base to mobile but
instead information on supervisory audio tones or similar means
may be transmitted from base to mobile.
The procedure for handoff from a digital channel of one base
station to an analog channel of an other base station is of course
a little more complex than the intracell handoff procedure because
more than one base station is involved. However, the normal
procedure of the system far handoff to a new base station and a new
analogue channel may be used, the procedure comprising transmit-
ting information to the mobile station on new base station and new
analog channel. If the invention is implemented in a mobile radio
system with analog channel handover according to the AMPS standard
then this procedure may be used. If the invention is implemented
in a system with different handover procedure then this handoff
procedure is well known to those skilled in the art. Anyone not
skilled in the art of procedures for analog channel handover is
recommended to study the specifications on the NMT and AMPS
systems or the specification of the system where the invention is
to be implemented and possibly also published patents in the field
of handover. Accordingly there is no need for describing such a
handover procedure here.
The :intelligence arid decision making in a mobile cellular system
according to figure 1 may be more or less centralized to the mobile
switch:i.ng centre or more or less decentralized to the base
stations. In a decentralized system more or less of the functions
of the mobile switching office during the handoff preparation and
performance may instead be performed in 'the responsible and/or
target base stations.



26
2p~~~°~
A method of directly estmating time dispersion and determining
when the time dispersion is too big will now be described in
connention with figures 6 and 8.
The signals from the A/D converter in figure 6 are sampled a
number, j, of times per bit time. In the equalizer the received
signals are correlated with. the locally stored time slot
indentifier. In this way an estimate Y(t) of the channel impulse
response is calculated in the equalizer and made available for.
processing and equalization in the central processor. In figure 8
the absolute values of Y(t) are indicated as a function of time.
The distance between adjacent indicated values of Y(t) is the
symbol (bit) time, T, divided by j. A process of estimating the
impulse response is described in detail in the earlier cited
article "Bit synchronization and timing sensitivity in adaptive
Viterbi Equalizers for Narrowband-TDMA Mobile Radio Systems".
Accordingly for a further understanding of figure 8 reference is
made to this article.
The equalizer correlates the received signal with the time slat
identifier TSID over a movable time span Wt, hereafter called the
measurement window, which is indicated in figure 8. The length of
the measurement window represent the amount of time dispersion
that the equalizer can handle e.g. energy within the measurement
window contributes in a positive manner for the demodulation/bit
detection process whereas energy outside this window act as an
interfering signal. The length of 'the gliding window is a a design
parameter in the construction of the equalizer. With a long window
the equalizer can handle more time dispersion but the complexity
arid power consumption will increase.
The equalizer moves the gliding window in relation to the impulse
response, calculates the impulse energy inside and places the
window in such a position in relation to the impulse response that
the part of the energy, c, inside the window is maximized. This
part of the impulse inside the window is shadowed in figure 8.
Maximizing the part of the energy inside means that the impulse
energy, r, outside the measurment window is minimized. The




27
positioning of the movable window is further elaborated in the
above mentioned reference. The estimation of the impulse response
is an inherent feature of the equalizer. Depending on the equa-
lization algorithm used, the impulse response may be estimated
only once in each burst, as described above, but may also be
updated during the duration time of the burst. Thus, the central
processor may obtain at least once every burst the amount of
useful energy , c, and the interfering energy, r, from the
equalizer.
The receiver processor can now average the individual c and r
values over a plurality, n, of bursts so that influence of estima-
tion errors, short time variations of radio propagation properties
etc is reduced. The averaged quantities of c and r are denoted C
and R respectively. Since C represents useful energy and R
represents disturbing energy the C/R ratio thus represents a
figure of merit of the ability of the equalization to reproduce
the transmitted data in a way similar to the well known S/t3 ratio
used for telecommunication in general.
If the C/R ratio is low the bit error rate will be high. An
criteria that can be used to determine if the time dispersion is
too severe to enable reproduction of an audio signal or data
transmitted :is to make the processor comparing 'the C/R with a
threshold, K. Thus if C/R is less than K, the processor has
determined that the time dispersion is so big that problem with
'the time dispersion currently exists.
Of course the described method of estimation of time dispersion on
a digital radio channel can be done either at 'the base station or
at the mobile station or at both. The invention is riot limited to
the use of this particular method but decisions to handover to an
other channel may be based on other methods of estimtion of time
dispersion.

Representative Drawing
A single figure which represents the drawing illustrating the invention.
Administrative Status

For a clearer understanding of the status of the application/patent presented on this page, the site Disclaimer , as well as the definitions for Patent , Administrative Status , Maintenance Fee  and Payment History  should be consulted.

Administrative Status

Title Date
Forecasted Issue Date 2001-08-14
(86) PCT Filing Date 1990-03-28
(87) PCT Publication Date 1990-11-01
(85) National Entry 1990-11-22
Examination Requested 1997-02-10
(45) Issued 2001-08-14
Deemed Expired 2008-03-28

Abandonment History

There is no abandonment history.

Payment History

Fee Type Anniversary Year Due Date Amount Paid Paid Date
Application Fee $0.00 1990-11-22
Registration of a document - section 124 $0.00 1991-05-10
Maintenance Fee - Application - New Act 2 1992-03-30 $100.00 1992-01-29
Maintenance Fee - Application - New Act 3 1993-03-29 $100.00 1993-01-22
Maintenance Fee - Application - New Act 4 1994-03-28 $100.00 1994-02-04
Maintenance Fee - Application - New Act 5 1995-03-28 $150.00 1995-03-01
Maintenance Fee - Application - New Act 6 1996-03-28 $150.00 1996-02-26
Maintenance Fee - Application - New Act 7 1997-04-01 $150.00 1997-02-13
Maintenance Fee - Application - New Act 8 1998-03-30 $150.00 1998-03-23
Maintenance Fee - Application - New Act 9 1999-03-29 $150.00 1999-03-19
Maintenance Fee - Application - New Act 10 2000-03-28 $200.00 2000-03-21
Maintenance Fee - Application - New Act 11 2001-03-28 $200.00 2001-03-14
Final Fee $300.00 2001-05-14
Maintenance Fee - Patent - New Act 12 2002-03-28 $200.00 2002-03-05
Maintenance Fee - Patent - New Act 13 2003-03-28 $200.00 2003-03-05
Maintenance Fee - Patent - New Act 14 2004-03-29 $250.00 2004-03-04
Maintenance Fee - Patent - New Act 15 2005-03-29 $450.00 2005-03-04
Maintenance Fee - Patent - New Act 16 2006-03-28 $450.00 2006-03-01
Owners on Record

Note: Records showing the ownership history in alphabetical order.

Current Owners on Record
TELEFONAKTIEBOLAGET LM ERICSSON
Past Owners on Record
DAHLIN, JAN ERIK AKE STEINER
UDDENFELDT, JAN ERIK
Past Owners that do not appear in the "Owners on Record" listing will appear in other documentation within the application.
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Document
Description 
Date
(yyyy-mm-dd) 
Number of pages   Size of Image (KB) 
Representative Drawing 2001-08-06 1 19
Representative Drawing 1999-07-27 1 27
Cover Page 2001-08-06 1 57
Description 2000-10-03 28 1,634
Description 1994-03-31 27 1,608
Abstract 1994-03-31 1 27
Claims 1994-03-31 5 253
Drawings 1994-03-31 7 180
Cover Page 1994-03-31 1 21
Claims 2000-10-03 18 696
Assignment 1990-11-22 5 192
Prosecution-Amendment 1997-02-10 2 141
Prosecution-Amendment 2000-06-05 1 36
Prosecution-Amendment 2000-10-03 22 818
Correspondence 2001-05-14 1 30
PCT 1990-11-22 43 2,372
Fees 1997-02-13 1 153
Fees 1996-02-26 1 64
Fees 1995-03-01 1 73
Fees 1994-02-04 1 66
Fees 1993-01-22 1 45
Fees 1992-01-29 1 37