`
`Petitioner Samsung - SAM1014
`
`
`
`FOR HE PURPOSES OF HVFORMAHON ONLY
`
`Codes used to identify States party to thel’CT onthe front pages ofpamphlets publishing internationalappli-
`cations under the PCT.
`
`AT Austria
`AU Australia
`BB Barbados
`BE Belgium
`BG Bulgaria
`BJ
`Benin
`BR Brazil
`CF Central African Republic
`CG Congo
`CH Switzerland —
`CM Cameroon
`DE Germany, Federal Republic of
`DK Denmark
`FI
`Finland
`
`,
`
`France
`Gabon
`United Kingdom
`Hungary
`Italy
`Japan
`Democratic People’s Republic
`of Korea
`Republic ofKorea
`Liechtenstein
`Sti Lanka
`Luxembourg
`Monaco
`Madagascar
`
`ML Mali
`MR Mauritania
`MW Malawi
`NL Netherlands
`NO Norway
`R0 Romania
`SD Sudan
`SE
`Sweden
`SN Senegal
`SU Soviet Union
`TD Cliad
`TG Togo
`US United States of America.
`
`2
`
`
`
`wo 39/03355
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`PCT/SE89/00049
`
`1
`
`Cellular digital mobile radio system with plural
`base station transmitters and method of transmitting
`information in such a system
`
`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
`
`analogue type,
`
`i.e. message information was transmitted in analogue form to
`
`and from mobile stations by transmitting and receiving analouge-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, Vortréige der NTG-Fachtagung vom 25.
`
`bis 27 November in Munich, "GLEICHKANALFUNKSYSTEME FUR DIE FRE-
`
`GJUENZUKONISCHE VERSORGUNG GROSSER GEBIETE" Berndt Heynisch pp
`
`41-46, VDE-VERLAG GmbH, Berlin, Elektrizitéitswirtschaft, Jg. 80(l98l), Heft
`
`6, pp 187-198 "C~)uasissynch1-oner Gleichwellenfunk-ein Gleichkanalfunk-Ver-
`
`fahren zur Erhbhung der Erreichbarkeit in Mobilfunknetzen.
`
`In known systems of the kind in question here, it is known to transmit message
`
`information from a central station or exchange to the base station transmitters
`
`either via cables or radio signals. It is also known to have equalisers in the fixed
`
`part of the mobile radio system for equalising differences in propagation time
`
`and attenuation in transmission from the exchange to the base station trans-
`
`mitters. The equalisers 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.
`
`3
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`wo 89/08355
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`PCVSE89/9°"497
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`2
`
`Digital mobile radio systems in which message information is transmitted
`
`digitally to and from mobile stations by transmission and reception of digitally 7‘
`
`’
`
`"
`
`‘modulated signals have been proposed in US 4675863 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 equalisers 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 equalisers in digital mobile radio
`
`systems can be mentioned: "Multi—path Equalization for Digital Cellular Radio
`
`Operation at 300 k. bit/5". K Raith, J—E Sjternvall and J Uddenfeldt, 36th IEEE
`
`Vehicular Technology Conference, pp 268-272, Dallas, Texas, USA May 1986.
`
`"Radio Test Performance of a Narrowband TDMA System", J-E Stjernvall, B.
`
`Hedberg, and S El-(mark, IEEE Vehicular Conference, Tampa, Florida, USA, June
`
`1937, RADIO TEST PERFORMANCE OF A NARROWBAND TDMA SYSTEM-
`
`DMS 90, J-E Stjervall, B. Hedberg, K Raith, T Béickstrfim and R Lofdahl.
`
`DISCLOSURE OF 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 informa-
`
`tion 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 equalisers
`
`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 of cell size to below what
`
`is
`
`necessary for reasons of capacity is that the number of handovers increases.
`
`4
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`WO 89/08355
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`PCT/SE89/00049
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`3
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`The object of the 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 invention, and particularly preferred embodiments thereof
`
`is disclosed in the independent and dependent claims. Somewhat simplified, it
`
`may be said that according to the invention there are utilised at least two base
`
`station transmitters 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 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 timezshift
`
`between the digitally modulated radio signals with the same message informa-
`tion 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 utilised 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
`
`5
`
`
`
`4
`
`particular mobile stations's position in relation to the base station transmitters.“
`
`The amount of tranmissions time shift can thus vary from mobile stat~ion.to§-"
`mobile station.
`
`According 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 utilised 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
`
`invention has time measurement means for estimating the mutual reception
`
`time shift in a mobile statioin 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 trans-
`
`mitter for the cell.
`
`In this embodiment
`
`the mobile radio system has trans-
`
`mission time shifting means for mutually time shifting the transmission times
`
`for base station transmitters of the same cell so that they transmit correspon-
`
`ding 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 means preferably include arrival time comparison means
`
`in at
`
`least certain mobile stations,
`
`for comparing the arrival
`
`times for
`
`corresponding digitally modulated signals transmitted from different bases
`station transmitters for the same cell.
`
`According to a somewhat different, preferred emdiment, the time measurement
`
`means include arrival time comparison means 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.
`
`6
`
`
`
`5
`
`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
`
`Figure 1 illustrates cells and positioning of transmitters in base stations in one
`
`embodiment of a mobile radio system in accordance with the invention.
`
`Figure 2 illustrates parts of a mobile radio system in accordance with the
`invention.
`
`BEST MODES FOR CARRYING OUT THE INVENTION
`
`A cellular mobile radio system in accordance with the 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 correspondence
`
`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.
`
`7
`
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`W0 89[08355
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`PCT/SE89[00049i -
`
`6
`
`Somewhat simplified, there is illustrated in Figure 1 the division of an area into 2
`
`cells and the assignation of base station transmitters to the cells ina mobile
`
`telephone system in accordance with the invention. For the sake of simplicity in
`
`Figure 1, all cells C1 to C24 are illustrated as regularhexagones 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 transmission to mobile
`
`stations where such transmission should from a purely geographically point" of
`
`view be performed by the base stations of a contiguous cell.
`
`For each cell Cl-C24 there is an ordinary base station transmitter BS1—BS24.
`
`For contiguous cells these transmitters are conventionally colocated 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
`
`the base station
`the cell C5. Correspondingly,
`station transmitter BS5 for
`transmitter B514 for the cell C14 is co-located with the base station trans-
`
`mitter BS16 for the cell C16 and with the base station transmitter B518 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 B52, B54 and B56 are co-located in the
`
`boundary areas between the cells C2, C4 and C6.
`
`Further to the ordinary base station transmitters BS1-B324 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 X56, X57, X510, X511,
`
`X513, XS15A, X51513, XS18A, XSlBB, X519, X52!) and XSZZA 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 XS18B for the cell C15 is co-located with the
`
`extra base station transmitter X519 for the cell C19 and the extra base station
`
`transmitter XS1BA for the cell C18. On the other hand, neither the extra base
`station transmitter X514 for the cell C14 nor the extra base station transmitter
`
`XSZZB for the cell C22 are co-located with any other base station transmitter,
`
`8
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`WO 89/03355
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`PCT/SE89/00049
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`7
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`but are situated approximately at the centre 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 am 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 transmitter 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 transmitter can be received without, or with a given time shift at
`
`the mobile station. The greater
`
`is
`
`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 Figure l, 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 hexagones. 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 of the
`
`radio signals. C is approximately 300000 km/s.
`
`In Figure 2 there are illustrated parts of a mobile radio system in accordance
`
`with the invention. A mobile radio exchange MSC is connected via cables L1,
`
`9
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`
`
`wo 39/03355
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`PC‘/SE39/“M9 if
`
`8
`
`m
`L2,....Lm, Ln to a plurality of basestations, of which two,-B and Bh are
`illustrated in the Figure.
`
`and-Lmb.lto
`The base station Bm has a central unit connected via cables L
`ma
`two transceiver units B
`and Bmb situated at a distance from the central unit;
`The central unit of the base station Bm includes a central line and control unit
`1, transmission time shifting means 2A and 2B, one for each of the transceivers,
`
`ma
`
`i
`
`reception time shifting means 3A and 3B, one for each of the transceivers and
`
`line units 4A and 4B, one for each of the transceivers.
`
`Both transceivers in the base station Bm are alike. Each such transceiver
`contains a line and control unit 5A or 58, transmitter means 6A or 6B, receiver
`
`means 7A or 7B, a transmission-reception filter BA or 8B and an antenna 9A or
`9B.
`
`5
`The base station Bn differs partly from station Bm primarily due to its central
`line and control unit 10 being situated in connection with one of its transceivers
`
`Bna. Accordingly, no cable with associated line units corresponding to Lm, Lmb,
`4A—5B is needed for the transceiver Bna, but only to the other transceiver Bnb.
`In addition, no transmission or reception time shifting means are included in any
`
`the corresponding means 2A, 2B, 3A and 3B are
`in B“, but
`central unit
`respectively included in transceivers Bna and Bnb.
`
`The mobile stations M51 and M52 are mutually alike. Each mobile station
`includes sound sensing means 11, encoding means 12, transmitting means 13,
`
`transmit-receive switch 14, reception means 15, equaliser and decoding means
`
`16, sound reproducing means 17, control means 18 and means 19 for feeding in
`
`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 transmision and reception time shifting
`
`means, the mobile radio system in Figure 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 Figure 2. One not
`
`10
`
`
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`11
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`WO 89/08355
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`PCI‘]SE89]00049"
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`10
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`in Bma time-shifted more or less before or after corresponding‘ radio signals
`from the antenna 9B in B rfib. In the preferred embodiment according to(F'igure7 ' ”
`2,
`the line and control unit 1 controls the varialbe delays in the transmission» '
`*
`
`time shifting means 2A and 2B so that the differences 4in;delay in thecables
`
`Lma and Lmb as well as the differences in the radio signal propagationtimes‘
`are counteracted. This may also be expressed by saying thatithe line and control
`
`;
`
`unit controls the variables delays in the transmission time shifting means 2A
`
`and 2B, such that the time shift of the radio waves on arrival at the mobile
`
`station is decreased compared if the means 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 Bma arrived at the
`antenna in M51, exactly simultaneously and in phase with corresponding radio
`signals transmitted from the antenna in Bmb. In practice, this is neither striven
`for nor achieved. Reflections occur in the propagation of the radio signals
`
`between the antennas, and the mobile station has an adaptive equaliser. 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
`
`means 2A and 2B. One method is to estimate in the fixed part of the mobile ~
`
`radio systemthe time shift between the mobile station radio signals at one of
`
`the transceivers Bma and the corresponding radio signals at the other trans-
`ceiver Bmb. There is
`thus obtained an estimation of
`the differences in
`propagation time to the mobile station,
`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
`
`Lma and Lmb and are not dependent on the position of the mobile station. In
`the embodiment according to Figure 2, this method can be applied in practice
`
`such that the delays in the reception time shifting means 3A and 3B are
`
`adjusted so that
`
`information received from the mobile station M51 at Bma
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`arrives at the line and control unit 1 simultaneously as corresponding informa-
`
`the line and
`tion received at Bmb from the mobile station MS1 arrives at
`control unit 1. The delays in the transmission time shifting means 2A and 2B are
`
`subsequently adjusted in correspondence with the optimum delays
`
`in the
`
`reception time shifting means 3A and 3B.
`
`The other method is to estimate in the mobile station the difference in arrival
`
`time or
`
`time shift between the digitally modulated radio signals from one
`
`transceiver Bma and the corresponding digitally mobile radio signals from the
`other transceiver Bmb. 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 synchronising words. These can be
`
`utilised if they are formed or supplemented so that two base station trans-
`
`mitters for
`
`the same cell do not only have identical synchronising words.
`
`7 Alternatively, radio signals digitally modulated with special synchronising words
`
`can be transmitted from the base station transmitters soleley 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 diffe-
`
`rence or time shift via radio signals to the fixed part of the system, where it is
`
`utilised for controlling the transmission time shifting means 2A and 2B. The line
`
`and control unit 1 then receives, via the respective line units 5A, 4A and 5B,
`
`4B,
`
`information about estimated arrival
`
`time difference from the mobile
`
`station in the same way as the line and control unit obtains message infor-
`mation 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
`
`Figure 2.
`
`Measuring the difference in arrival time or time shifting for correspoding radio
`
`signals can be performed in a conventional way, e.g. with the aid of correlation.
`
`In the cases where the radio signals conventionally contain predetermined
`
`synchronising patterns (words),
`
`the time difference between these patterns
`
`(words)
`
`in different signals can be measured using conventional methods. A
`
`mobile station control means 18 and/or a base station line and control unit 1,9
`
`possible in combination with the transceivers‘ line units 5A and 58 can then
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`12
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`comprise time measurement means for estimating receptions tirnefshifting.or
`
`the arrival time comparison means for comparing arrival times.
`
`When so required, a base station preferably utilises conventionally the same
`
`transmitter means 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 transmitter 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 Bma,
`Bmb and Bria’, Bnb of the base stations in a mobile radio system according to
`Figure 2. The transmission time shifting means are then controlled to a
`
`balancing state where the delay of information from the line and control unit 1
`
`to the antenna in one transceiver Bma is equally as great as the delay of *
`information from the line and control unit 1 to the antenna in the other
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`transceiver Bmb. The corresponding situation can apply when a base station‘
`"listens" in unoccupied combinations of time slot and frequency range channel
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`for calls from mobile stations of unknown positions relative to the transceivers
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`of the base stations. The reception time shifting means 3A and 3B can then be
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`controlled to a balancing state where the delay of the message information
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`from the antenna in one transceiver Bna to the line and control unit 9 is equally
`as great as the delay of information from the antenna in the other transceiver
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`Bnb to the line and control unit 9.
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`The mobile Stat‘-i0n8 M51 and M52 have adaptive equalisers, 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
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`during a reception time interval. In known cellular digital mobile radio systems
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`with only one base station transmitter per cell, the reception time interval of
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`the equalisers is dimensioned according as the dispersion on the radio channel,
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`i.e. expected time shifts between corresponding signals from a single base
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`station transmitter due to multipath propagation and reflections. Because of
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`the equaliser, not only the radio signal having the greatest amplitude or arriving
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`first to the mobile station is utilised for reconstructing the digital modulation,
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`but. also other corresponding radio signals arriving with a time shift within the
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`extent of the equaliser‘s reception time interval can be utilised. The mobile
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`stations in a system in accordance with the invention have equalisers which are
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`dimensioned such that the reception time interval of the mobile station in
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`reconstruction of the digital modulation is greater than the time it takes for
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`signals to propagate a distance as long as the greatest distance between two
`base station transmitters associated with the same cell within a restricted
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`geographical area. With the base station transmitters placed according to
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`Figure 1, and the restricted area being the area composed of the cells C1 to
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`C24, the mobile station equalisers would thus be dimensioned for a reception
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`inverval of reconstruction which is greater than 2L/c. Taking into account that
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`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
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`the propagation time from another base station transmitter for the same cell,
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`the reception time interval of the mobile stations is prefereably substantially
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`greater than the time it takes for radio signals to propagate a distance which is
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`just as great as the greatest distance between two base station transmitters
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`associated with the same cell within the geographical area in question.
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`In prior art mobile radio systems it is known to have cells of small size in city
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`areas or densely populated areas with many calls per hour and to have cells of
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`large size in rural areas with few calls per hour. Outside the restricted
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`geographical area composed of cells C1 to C24 in figure 1 there might be cells
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`of greater size than that of C1 to C24. Occasionally such cells may have plural
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`base stations transmitting the same information for overcoming radio shadows
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`from mountains. A mobile radio according to the present invention may be used
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`outside the restricted area and in such a cell but the advantages conferred by
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`the invention can not be expected if the transmission distance between the
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`transmitters assigned to such a cell is too great in relation to the reception
`time interval.
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`In mobile radio systems in accordance with the invention, the modulation time
`
`interval of the digital modulation of the signals can be of the same order, of
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`magnitude as the time it takes for radio signals to propagatefacfiistanceajust as
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`long as the greatest transmitting distance between two base’ stat-ion-‘trans-’—’
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`mitters serving thesame cell within a particular-‘-area. Although thevinventiorr
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`affords greater advantages the smaller the modulation time‘ interval
`
`is in“
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`relation to this propagation time, and the invention has its greatest importance" a
`
`when the modulation time interval is less than the mentioned propagation time,‘
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`the invention can mean substantial advantages even when the modulation time.
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`interval is some few times greater than the mentioned propagation time.
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`It is conceivable to use different kinds of digital modulation in a mobile radio
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`system, whereby somewhat different relationships can exist between infor-
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`mation transmission rate and modulation time interval. In digital modulation of
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`the radio signals involving transmission of one symbol at a time of a sequence
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`of uncorrelated symbols, the modulation time interval will be the time during
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`which a single symbol is decisive for the modulation. For example, if a sequence
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`a of binary symbols individually and one at a time determine the modulation, the
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`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
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`interval will be the inverted value of the transmission rate in bits. In digital
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`modulation of the signals involving two or more at a time of a sequence of
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`digital symbols being decisive for the modulation during wholely or partly
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`overlapping times, the modulation interval can be the time when a preceeding,
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`but not the nearest subsequent symbol affects the modulation. For example, in
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`digital modulation according to SE 8102802-9, Figures 1-2, a symbol affects the
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`phase of a carrier wave during a time interval 3T. The nearest preceeding
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`symbol also affects the phase change of the carrier wave during a first part 2T
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`of the time i