lllllllllllllllllIllll||l||Illll|l|l||||||Illllllllllllllllllllllllllllllll
`
`U5005109528A
`
`United States Patent
`
`[19]
`
`[11] Patent Number:-
`
`5,109,528
`
`Apr. 28, 1992
`[45] Date of Patent:
`Uddenfeldt
`
`
`[54] HANDOVER METHOD FOR MOBILE
`. RADIO SYSTEM
`
`OTHER PUBLICATIONS
`
`[75]
`
`Inventor:
`
`Jan-Erik Uddenfeldt,V'allingby,
`Sweden
`
`[73] Assignee: Telefonaktiebolaget L M Ericsson,
`Sweden
`
`[21] App]. No.: 365,432
`
`[22] Filed:
`
`Jun. 13, 1989
`
`Foreign Application Priority Data
`[30]
`Jun. 14, 1988 [SE]
`Sweden ................................ 8802298
`
`Int. Cl.5 .............................................. H04Q 7/00
`[51]
`[52] U.S. C1. ...................................... 455/333; 379/60
`[58] Field of Search ....................... 455/33, 34, 51, 54,
`455/56; 379/59, 60
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`.
`4,211,894 7/1980 Watanabe et al.
`4,596,042
`6/1986 Stangel .................................. 455/56
`
`4,697,260 9/1987 Grauel et al.
`.
`
`4,718,081
`1/1988 Brenig ..........
`
`4,723,266
`2/1988 Perry ............
`..
`
`4,737,978 4/1988 Burke et a1.
`.
`. 379/60
`4,759,051
`7/1988 Han ..................
`.. 379/59
`4,955,082 9/1990 Hattori et a1.
`........................ 455/33
`
`FOREIGN PATENT DOCUMENTS
`
`0274857
`
`7/1988 European Pat. Off.
`
`.
`
`“User Access in Portable Radio Systems,” Bernhardt,
`Jun. 1987, Bell Research Inc.
`1
`“RF Performance of Macroscopic Diversity in Univer-
`sal Digital Portable Radio Communications,” Bern-
`hardt, Jun. 1986, IEEE Conference.
`Japanese Patent Abstrct vol. 9, No. 169 (E-328).
`
`Primary Examiner—Curtis Kuntz
`Attorney, Agent, or Firm—Burns, Doane, Swecker &
`Mathis
`
`ABSTRACT
`[57]
`The invention relates to a method in mobile radio sys-
`tems in which the responsibility for transmitting mes-
`sage information to a mobile station is handed over from
`a first base station transmitter to a second base station
`transmitter. In this method, the same radio channel is
`used, if possible, before and after the handover for trans-
`mitting message information to the mobile station. If the
`same radio channel can be utilized, the handover takes
`place without the mobile station being informed before-
`hand of the handover by a special signal or order or the
`like. In digital mobile ratio systems with digital trans-
`mission of message information by digital modulation of
`the radio signals, the transmission is preferably started
`from a second base station transmitter before the trans-
`mission is terminated from a first base station transmit-
`ter. During a certain transmission time substantially the
`same message information is transmitted to the mobile
`station from both the first and second base station trans-
`mitters.
`
`6 Claims, 2 Drawing Sheets
`
`
`
`CLEARWIRE 1012
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`

`

`US. Patent
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`Apr. 28, 1992
`
`Sheet 1 of 2
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`5,109,528
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`

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`US. Patent
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`Apr. 23, 1992
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`Sheet 2 of 2
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`5,109,528
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`AIM/[f
`1740/0
`[Ml/AM?
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`

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`1
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`5,109,528
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`2
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`HANDOVER METHOD FOR MOBILE RADIO
`SYSTEM
`
`TECHNICAL FIELD
`
`The present invention relates to a mobile radio sys-
`tem which has a plurality of radio channels for transmit~
`ting control information and message information be-
`tween base stations and mobile stations. More specifi-
`cally, the invention relates to a method in which the
`responsibility for transmitting message information to a
`mobile station is handed over from at least a first base
`station transmitter to at least a second base station trans-
`mitter.
`
`BACKGROUND ART
`
`It is desirable that a mobile radio system has high
`traffic handling capacity and a high degree of coverage.
`The traffic handling capacity of a mobile radio system is
`dependent,
`inter alia, upon the number of available
`radio channels and how effectively these channels may
`be utilised. It is known to arrange several base stations
`with small covering areas close to each other in a mo-
`bile radio system. Available radio channels can then be
`utilised in a more efficient way for handling peak traffic
`within a limited geographical area than if base stations
`with large covering areas are arranged far away from
`each other in the mobile radio system. Arranging sev-
`eral base stations close to each other can also increase
`the accessibility in a mobile radio system, particularly if
`the covering areas of the base stations are mutually
`overlapping. Two base station with such overlapping
`covering areas cannot, however, normally use the same
`radio channel for communication with different mobile
`stations.
`When a mobil station moves from the area covered
`by one base station to the area covered by another base
`station in a mobile radio system, the responsibility for
`communication with the mobile must be transferred
`from one base station to another. Such transfer in con-
`juction with an established connection to the mobile
`station is usually called “handover".
`Handover can be a critical function in known tradi—
`tional mobile radio systems. There is a risk of the estab-
`lished connection being temporarily interrupted or
`completely lost
`in conjunction with the handover.
`There is usually required comparatively extensive sig-
`nalling in the mobile radio system in conjunction with
`changing of radio channel at handover. The degree of
`coverage of a mobile radio system depends, inter alia,
`on the presence of radio shadows and how the covering
`areas of the individual base stations overlap each other.
`The possibility of establishing new connections and
`maintaining established connections to/from mobiles
`also depends on the presence of reflections and interfer—
`ence.
`
`10
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`15
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`20
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`25
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`30
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`35
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`40
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`50
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`55
`
`For increasing the degree of coverage in a mobile
`radio system, it is known to transmit substantially the
`same information to a mobile from two or more base
`transmitters and to receive information from a mobile at
`two or more base receivers. The publication “RF PER-
`FORMANCE OF MACROSCOPIC DIVERSITY IN
`UNIVERSAL PORTABLE RADIO COMMUNICA-
`TIONS:
`FREQUENCY REUSE CONSIDER-
`ATIONS” by Richard Cl Bernhardt, from IEEE Inter-
`national Conference on Communications, Jun. 22-25,
`1986, Toronto, Canada describes different types of cells
`and placing of transmitters and the result of simulations
`
`65
`
`of such a system. The publication “USER ACCESS IN
`PORTABLE RADIO SYSTEMS IN THE NOISE
`LIMITED ENVIRONMENT" by Richard C. Bern-
`hardt, from ICC’87, Jun. 1987, Seattle, Wash., U.S.A.,
`describes different types of cell and placing of transmit-
`ters as well as comparsions between different criteria
`and algorithms for the selection of base transmitter.
`US. patent application Ser. No. 07/315,561 describes
`a mobile radio system and a method of transmitting
`message information where at least two bases at least
`partially simultaneously transmit substantially the same
`message information to a mobile. The object with this is,
`inter alia, to enable the degree of coverage to be made
`greater without the cells in a mobile radio system need-
`ing to be reduced, which gives greater freedom in the
`selection of cell pattern, and fewer handovers. The
`intention is that the mobile radio system shall be given
`better possibilities of establishing new connections and
`maintaining already established connections. According
`to the patent application, these objects are achieved by
`certain relationships between modulation of radio sig-
`nals and parameters of adaptive equalisers in the mo-
`biles and the distance between base transmitters which
`transmit the same message information.
`in cellular mobile radio systems available channels
`may be allocated to cells and mobiles in various ways.
`In systems with fixed channel allocation a base for a
`particular cell may only use certain predetermined
`channels. Bases in adjacent cells use different channels.
`Bases in different cells sufficiently distant from each
`other may use same channels according to a channel
`re-use or allocation plan. In systems with adaptive chan-
`nel allocation some or all channels are a common re-
`source to all base stations, which means that some or all
`channels may be used by any base as long as the radio
`transmission conditions permit,
`i.e. certain carrier to
`interference or noise level.
`
`SUMMARY OF THE INVENTION
`
`At least in certain mobile radio systems, it is a prob-
`lem to make the handover so that the risk of temporary
`interruption in a connection or that the connection is
`lost in conjunction with handover is sufficiently small.
`This problem is particularly great with peak traffic.
`Another problem in connection with handover is the
`comparatively large need for signalling in the mobile
`radio system. The object of the invention is, inter alia, to
`ameliorate these problems.
`One object of the present invention is to achieve a
`handover where the risk of temporary interruption or
`lost connections can be sufficiently small.
`Another object of the present invention is to achieve
`a handover where the need of signalling via radio sig-
`nals in the mobile radio system may be comparatively
`small.
`A still further object of the present invention is to
`achieve a handover which is particularly suitable for
`mobile radio systems where two or more base transmit-
`ters spaced from each other transmit essentially similar
`message information to a mobile.
`Yet another object of the invention is to achieve a
`handover which is particularly suitable for mobile radio
`systems with a high degree of coverage and large over-
`lapping between the covering areas of different base
`transmitters.
`In a method in accordance with the invention the
`responsibility of transferring message information to a
`
`

`

`10
`
`40
`
`3
`is handed over from at least one first
`mobile station,
`base transmitter to at least a second base transmitter.
`Distinguishing for a method in accordance with the
`invention, and particularly preferred embodiments
`thereof will be seen from'the independent and the de» 5
`pendent claims. Somewhat simplified,
`it may be said
`that in a method in accordance with the invention, the
`same radio channel is preferably used for transmitting
`message information to the mobile before and after the
`handover.
`the
`The handover preferably takes place without
`mobile being informed beforehand that the handover is
`going to take place. According to a further preferred
`embodiment of the invention, the radio transmission of
`message information from a second base station trans- 15
`mitter to the mobile station is started before transmis-
`sion of message information to the mobile from a first
`base station transmitter is terminated. Essentially the
`same message information is thus transmitted during a
`transition time to the mobile both from a first and a 20
`second base station transmitter.
`In mobile radio systems where at least two base sta—
`tion transmitters are simultaneously responsible for
`transmitting message information to at
`least certain
`mobiles, it may be preferred not to start or interrupt the 25
`transmission from several base transmitters simulta-
`neously in conjunction with the handover. The trans-
`mission of message information to the mobile is then
`preferably started from one of the second base transmit-
`ters before initiating transmission of message informa- 30
`tion from another of the second base transmitters. In
`such a case, the transmission of message information to
`the mobile from one of the first base transmitters is
`preferably terminated before the transmission of such
`information is terminated from another of the first base 35
`transmitters. Initiation and termination of the transmis-
`sion from the base transmitters preferably takes place in
`an order such that at least two base transmitters transmit
`essentially the same message information to the mobiles
`the whole time.
`In the cases where transmission from a second base
`transmitter must take place on another radio channel
`than the transmission from a first base transmitter, it is
`preferable that the radio channels are selected such that
`during the transition time the mobile station can receive 45
`the transmissions from the different base transmitters
`essentially without interference from each other. De-
`signing the handover of responsibility for transmitting
`message information to a mobile in accordance with the
`invention means several advantages, which are accentu- 50
`ated in preferred embodiments.
`Perhaps the most important advantage is that in the
`cases where the same radio channel is utilised by the
`bases before, during and after the handover, the need
`for handover signaling via radio signals can be made 55
`particularly small. If the mobile sends and receives on
`the same channels before, during and after the hand-
`over, it does not really need to know whether the re-
`sponsibility for communication has been handed over.
`In an extreme case, there is thus no need of handover 60
`signalling at all via radio signals in conjunction with
`handover, and all handover signalling could take place
`in the stationary part of the mobile radio system. An-
`other important advantage is that the risk of temporary
`interruption in an established connection call to a mo- 65
`bile or that the connection is lost can be made small.
`Another advantage is great flexibility with regard to
`distributing tasks between the mobile and base in con-
`
`5,109,528
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`4
`
`junction with the handover. Preparations for and initia-
`tive to the handover can take place to a greater or lesser
`extent in the mobile. It will then be possible to form and
`distribute tasks in conjunction with preparation for,
`initiation and execution of the handover such that the
`need of signalling with respect to the handover will be
`comparatively small.
`A still further important advantage with the method
`in accordance with the invention is that it is particularly
`suitable in a mobile radio system where at least two base
`station transmitters are simultaneously responsible for
`transmitting message information to at
`least certain
`mobiles. In such a system the increasing degree of cov-
`erage can be utilised to the full in handing over the
`responsibility in accordance with the invention.
`Further advantages ought to be understood by one
`skilled in the art after having studied the description of
`preferred embodiments.
`BRIEF DESCRIPTION OF DRAWINGS
`
`FIG. 1 illustrates cells and location of base station
`transmitters in a mobile radio system where in certain
`areas two or three base station transmitters simulta-
`neously transmit message information to at least certain
`mobiles.
`FIG. 2 illustrates two mobiles and some stationary
`parts of a mobile radio system according to FIG. 1.
`BEST MODES FOR CARRYING OUT THE
`INVENTION
`
`A method in accordance with the invention is partic-
`ularly suitable for a mobile radio system where at least
`two base station transmitters are simultaneously respon-
`sible for transmitting message information to at least
`certain mobile stations. It seems therefore to be suitable
`*to describe an embodiment of a method in accordance
`with the invention in connection with such a system.
`A cellular mobile radio system illustrated in FIG. 1
`has mobile and base stations with transmitters and re—
`ceivers for radio signals. Message information is trans—
`mitted digitally to and from" the mobiles by transmission
`and reception of radio signals with digital modulation
`corresponding to the message information. The radio
`signals are sent on one of a plurality of radio channels.
`Several radio channels can share in time multiplex the
`same radio frequency, and radio signals can be transmit-
`ted to and from several mobiles in different time slots on
`the radio frequency, these slots being associated with
`the radio channels.
`The mobiles may move within and between the cells
`of the system. The base transmitters are assigned to the
`cells so that there is at least one base transmitter for
`each cell, for transmitting signals to the mobiles of the
`cell.
`
`There is no completely fixed channel allocation for
`all cells and base stations in the entire system. At least
`for some base stations and cells there is allowed some
`freedom in the selection of radio channels. Thus the
`channel allocation in the system is some kind of dy—
`namic or adaptive channel allocation meaning that at
`least some channels are a common resource to at least
`some neighbour bases/cells. Somewhat simplified, there
`is illustrated in FIG. 1 the division of an area into cells
`and the assignation of base station transmitters to the
`cells in a mobile telephone system. For the sake of sim-
`plicity in FIG. 1, all cells C1 to C24 are illustrated as
`regular hexagons with sides L. In practice, the cells will
`probably have different sizes and shapes. In addition,
`
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`5,109,528
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`5
`depending on traffic conditions, it will often be suitable
`with overlapping in the boundary areas between the
`cells. To a certain extent, the base transmitters can then
`off-load each other by handling transmissions to mo-
`biles where such transmission should from a purely
`geographically point of view, he performed by the base
`transmitters of a contiguous cell.
`For each cell C1—C24 there is an ordinary base trans—
`mitter B81—BSZ4. For contiguous cells these transmit-
`ters are conventionally co-located in groups of three.
`For example, the base transmitter 881 for the cell C1 is
`co-located with the base transmitter B83 for the cell C3
`and with the base transmitter B85 for the cell C5. Cor-
`respondingly, the base transmitter B814 for the cell C14
`is co-located with the base transmitter 8816 for the cell
`C16 and with the base transmitter B818 for the cell C18.
`These co-located ordinary base station transmitters are
`situated in the boundary regions between the cells to
`which they are assigned. For example, the ordinary
`base transmitters B82, B84 and BS6 are co-located in
`the boundary areas between the cells C2, C4 and C6.
`Further to the ordinary base transmitters BSl-B824
`the system includes a number of extra base transmitters
`for certain of the cells. Cells C6, C7, C10, C11, C13,
`C14, C19 and C20 each has one extra base station trans-
`mitter. For each of the cells C15, C18 and C22 there are
`two extra base transmitters. Of the extra base transmit-
`ters X86, X87, X810, X811, X813, XSlSA, XSlSB,
`X818A, XSl8B, X819, X820 and X822A are co-located
`in groups with three extra base transmitters in each
`,group in a similar manner as the ordinary base transmit-
`ters. Accordingly, for example, the extra base transmit-
`ter XSlSB for the cell C15 is co-located with the extra
`base transmitter X819 for the cell C19 and the extra
`base transmitter X818A for the cell C18. On the other
`hand, neither the extra base transmitter X814 for the
`cell C14 nor the extra base transmitter X822B for the
`cell C22 are co-located with any other base transmitter,
`but are situated approximately at the centre of the cell
`with which they are associated. An extra base transmit-
`ter does not need to differ technically from an ordinary
`base transmitter. For a given cell an extra base transmit-
`ter can thus have technical equipment of the same type
`as an ordinary base transmitter for the same cell. In
`principle, it can also function in the same way as the
`ordinary one. If there are two identical base station
`transmitters for a given cell, either of them may be
`respectively regarded as ordinary or extra in certain ,
`cases.
`50
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`ters associated with the cell, the greater in general can
`be the time shift. When the ordinary base station trans—
`mitters and the extra base station transmitters are situ—
`ated according to FIG. 1, the distance between two
`base transmitters for the same cell varies between L and
`2L, i.e. between the side and diameter of the regular
`hexagons. If, for the sake of simplicity, reflections are
`ignored and the assumption made that the base transmit-
`ters transmit without mutual
`time shifting,
`the time
`reception shift at the mobile station could then attain a
`maximum of 2L/c, where c is the propagation rate of
`the radio signals.
`In FIG. 2 there are illustrated parts of a mobile radio
`system in accordance with FIG. 1. A mobile radio ex-
`change MSC is connected via cables L1, L2, .
`.
`. Lm, L,
`to a plurality of bases of which two, Em and B” are
`illustrated in FIG. 2. The base Bm has a central unit
`connected via cables Lma and Lmb to two transceiver
`units Em and me situated at a distance from the central
`unit. The central unit of the base Bm includes a central
`line and control unit 1, transmission time shifting means
`2A and 2B, one for each of the transceivers, reception
`time shifting means 3A and 3B, one for each of the
`transceivers and line units 4A and 4B, one for each of
`the transceivers.
`Both transceivers in the base Bm are alike. Each such
`transceiver contains a line and control unit 5A or 5B,
`transmitter means 6A or 6B, receiver means 7A or 7B,
`a transmission-reception filter 8A‘or 8B and an antenna
`9A or 9B.
`
`The base B,, differs partly from base Bm, primarily due
`to its central line and control unit 10 being situated in
`juxtaposition with one of its transceivers BM. Accord-
`ingly, no cable with associated line units corresponding
`to Lm, Lmb, 4A—5B is needed for the transceiver BM,
`but only for the other transceiver Bnb. In addition, no
`transmission or reception time shifting means is in-
`cluded in any central unit in B”, but the corresponding
`means 2A, 2B, 3A and 3B are respectively included in
`transceivers B”a and Bnb.
`The mobiles MS] and M82 are mutually alike. Each
`mobile includes sound sensing means 11, encoding
`means 12,
`transmitting means 13,
`transmit-receive
`switch 14, reception means 15, equaliser and decoding
`means 16, sound reproducing means 17, control means
`18 and means 19 for feeding in and out or presentation
`of digital information.
`Apart from the bases having two transceiver units at
`a distance from each other, and having controllable
`transmission and reception time shifting means,
`the
`mobile radio system in FIG. 2 functions in most respects
`in a way well-known in mobile radio systems. No com-
`plete description of how the system functions in differ-
`ent respects should therefore be necessary for one
`skilled in the art, and it should only be necessary to
`describe what is unique or unusual in the mobile radio
`system according to FIG. 2. One not skilled in the art of
`mobile radio systems is referred to the technical litera-
`ture‘.
`
`Message information e.g. speech or data that the
`mobile radio exchange forwards towards a mobile in the
`cell Cm, e.g. the M81, is transmitted from the mobile
`radio exchange via the cable L”, to the line and control
`unit 1. From here the information is transferred via the
`transmission time shifting means 2A, line unit 4A, cable
`Lma and line and control unit 5A to the transmitting
`means 6A. The transmitting means 6A transmits, via the
`transmission-reception filter 8A and antenna 9A, radio
`
`The extra base transmitter or transmitters for a given
`cell transmit radio signals which are substantially the
`same as those sent by the ordinary base transmitter of
`the cell. The signals are digitally modulated with selec-
`tive digital message information to the individual mo-
`biles in the cell. The radio signals may also be digitally
`modulated with general control information common to
`all mobiles concerned. A mobile in a cell for which
`there is One or more extra base transmitter can therefore
`receive, at least in certain cases, corresponding radio
`signals from more than one base transmitter approxi-
`mately simultaneously within the same frequency
`range. Depending on the mutual, relative positions in
`the cell of the mobiles and bases as well as the transmis-
`sion times and propagation paths of the radio signals
`from the base transmitters to the mobile stations, corre-
`sponding signals from different base transmitters can be
`received without, or with a given time shift at the mo-
`bile. The greater the distance between the base transmit-
`
`55
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`5,109,528
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`signals with digital modulation in correspondance with
`the message information from the mobile radio ex
`change.
`The message information from the mobile radio ex-
`change is also transferred from the line and control unit
`1 via the transmission time shifting means 28, line unit
`4B, cable Lmb and line and control unit SB to the trans-
`mission means 6B in the transceiver me. The transmis—
`sion means (SB transmits, via the transmission reception
`filter 8B and antenna 9B, radio signals with digital mod-
`ulation in correspondance with the information from
`the mobile radio exchange.
`Depending on the delay in transferring the message
`information to the transmission means 6A and the corre-
`sponding delay in transferring to the transmission means
`6B the radio signals can be transmitted from the antenna
`9A of the transceiver unit Bma substantially without
`time shifting, or time shifted in relation to the transmis-
`sion of corresponding radio signals from the antenna 9B
`of the other transceiver me.
`The radio signals from the antenna 9A i B,,m arrive at
`a given mobile in the cell Cm, e.g. the mobile MS], with
`or without time shift in relation to corresponding radio
`signals from the antenna 9B in me. The possible time
`shift on arrival at the mobile depends partly on possible
`time shifting at
`transmission from the antennas and
`partly on possible difference in propagation time for the
`radio waves from the antennas. The transmission time
`shifting means 2A and 2B have a variable delay and can
`be controlled by the line and control unit 1, such that
`the signals are transmitted from the antenna 9A in B"m
`time-shifted more or less before or after corresponding
`signals from the antenna 9B in BM. In the preferred
`embodiment according to FIG. 2, the line and control
`unit 1 controls the variable delays in the transmission
`time shifting means 2A and 2B so that the differences in
`delay in the cables LM and Lmb as well as the differ-
`ences in the radio signal propagation times are counter—
`acted. This may also be expressed by saying that the line
`and control unit controls the variable delays in the
`transmission time shifting means 2A and 2B, such that
`the time shift of the radio waves on arrival at the mobile
`is decreased compared with the situation where the
`means 2A and 2B have the same fixed delay. It could be
`thought that the ideal case were that the line and con-
`trol unit controlled the delays in the time shifting means
`2A and 28 so that the digitally modulated signals trans-
`mitted from the antenna in Bm arrived at the antenna in
`MS], exactly simultaneously and in phase with corre-
`sponding signals transmitted from the antenna in BM.
`In practice,
`this is neither striven for nor normally
`achieved. Reflections occur as the radio signals are
`propagated between the antenna, and the mobile has an
`adaptive equaliser. It is therefore not necessary for the
`signals from the different transceiver units to arrive
`exactly simultaneously to the mobile. On the contrary,
`there is preferably sought a small time shift to achieve
`diversity against Rayleigh fading. One not skilled in this
`art and who is desirious of obtaining further information
`can find it in the technical literature and in publications
`such as: “Radio Test Performance of a Narrow-band
`TDMA System-DMS 90", J-E Stjernvall, B. Hedberg,
`K. Raith, T. Backstrom and R. Lofdahl.
`In principle, there are at least two conceivable meth-
`ods of determining how the line and control unit 1 shall
`control the delay in the transmission time shifting means
`2A and ZB. One method is to estimate in the fixed part
`of the mobile radio system the time shift between the
`
`8
`mobiles radio signals at one of the mobile radio system
`the time shift between the mobiles radio signals at one of
`the transceivers BM and corresponding radio signals at
`the other transceiver me. There is thus obtained an
`estimation of the differences in propagation time to the
`mobile, these differences depending on the position of
`the mobile. Remaining differences in delay are related
`to the fixed part of the mobile radio system, e.g. differ—
`ences in length of the cables Lmu and Lmb and are not
`dependent on the position of the mobile. In the embodi-
`ment according to FIG. 2, this method can be applied in
`practice such that the delays in the reception time shift—
`ing means 3A and 3B are adjusted so that information
`received from the mobile MS] at Bma arrives at the line
`and control unit 1 simultaneously as! corresponding
`information received at me from the mobile station
`MS] arrives at the line and control unit 1. The delays in
`the transmission time shifting means 2A and 2B are
`subsequently adjusted in correspondence with the opti-
`mum delays in the reception time shifting means 3A and
`3B. The other method is to estimate in the mobile the
`difference in arrival time or time shift between the digi-
`tally modulated radio signals from one transceiver B,"a
`and the corresponding digital signals from the other
`transceiver me. 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 with
`additional individual wards so that two base transmit-
`ters for the same cell do not only have identical syn-
`chronising words. Alternatively, special synchronising
`words can be transmitted from the base station transmit-
`ters solely to enable the mobile to estimate the differ-
`ences in arrival times or time shifts. The mobile trans-
`mits information about the estimated arrival time differ-
`ence or time shift via radio signals to the fixed part of
`the system, where it is utilised for controlling the trans-
`mission time shifting means 2A and 28. The line and
`control unit 1 then receives, via the respective line units
`5A, 4A and SB, 4B, information about estimated arrival
`time difference from the mobile in the same way as the
`line and control unit obtains message information from
`the mobile.
`Itis conceivable per se, but hardly to be preferred, to
`combine both methods for controlling the transmission
`time shift in a mobile radio system according to FIG. 2.
`Measuring the difference in arrival time or time shift—
`ing for corresponding radio signals can be performed in
`a conventional way, e.g. with the aid of correlation. In
`the cases where the radio signals conventionally contain
`predetermined synchronising patterns (words), the time
`difference between the appearance of these in different
`signals can be measured using conventional methods. A
`mobile control means 18 and/or a base line and control
`unit 1 or 10, possible in combination with the transceiv-
`er’s line units 5A and 5B can then include time measure»
`ment means for estimating reception time shifting, or
`arrival time comparison means for comparing arrival
`times.
`When so required, a base preferably conventionally
`utilises the same transmitter means and antenna for
`transmitting,
`in time multiplex within the same fre-
`quency range, radio signals digitally modulated with
`message information to different mobiles associated
`with the same cell. Radio signals with message informa-
`tion to a given mobile are then transmitted from differ-
`ent base transmitters with a possible transmission time
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`65
`
`

`

`5,109,528
`
`9
`shift which is specially adjusted with regard to the
`position of thisparticular mobile. The case can arise
`where a base in a mobile radio system needs to transmit
`a radio signal with information other than message in-
`formation intended for reception by several or all of the
`mobiles in the cell, e.g. information as to the identity of
`the base/cell. Such signals are preferably transmitted
`simultaneously without mutual time shifting, from the
`transceivers Bma, me and BM, Bnb of the base in a
`mobile radio system according to FIG. 2. The transmis-
`sion time shifting means are then controlled to a balanc-
`ing 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 trans-
`ceiver me. The corresponding situation can apply
`when a base “listens” in unoccupied combinations of
`time slot and frequency range for set up calls from
`mobiles in unknown positions relative to the transceiv-
`ers of the base. The reception time shifting means 3A
`and 3B can then be controlled to a balancing state
`where the delay of the message information from the
`antenna in one transceiver BM to the line and control
`unit 9 is equally as great as the delay of information
`from the antenna in the other transceiver Bnb to the line
`and control unit 9.
`The mobiles M51 and MS; have adaptive equalisers,
`whereby the digital modulation during a modulation
`time interval in the radio signals transmitted from a base
`transmitter can be reconstructed from signals received
`during a reception time interval. In known cellular,
`dig