a2, United States Patent
`US 6,208,863 B1
`(10) Patent No.:
`Mar.27, 2001
`(45) Date of Patent:
`Salonaho
`
`US006208863B1
`
`(EP).
`3/1994.
`0 589 278 A3
`(54) HANDOVERIN A MOBILE
`COMMUNICATION SYSTEM HAVING A
`3/1994 (EP).
`589 279
`MULTILAYER RADIO COVERAGE
`3/1996.
`(EP).
`701 382
`2240696=8/1991 (GB).
`WO 92/01950
`—-2/1992
`(WO).
`(75)
`Inventor: Oscar Salonaho, Helsinki (FD)
`
`(73) Assignee: Nokia Telecommunications Oy, Espoo
`(FD)
`
`(*) Notice:
`
`Subject to any disclaimer, the term ofthis
`patent is extended or adjusted under 35
`US.C. 154(b) by 0 days.
`
`(21) Appl. No.:
`
`08/981,625
`
`(22) PCT Filed:
`
`Apr. 14, 1997
`
`(86) PCT No.:
`
`PCT/FI97/00227
`
`§ 371 Date:
`
`Dec. 16, 1997
`
`§ 102(e) Date: Dec. 16, 1997
`
`OTHER PUBLICATIONS
`
`Steele et al: “Teletraffic performance of microcellular per-
`sonal communication networks”, IEEE Proceedings-I,vol.
`139, No. 4,Aug. 1992, pp. 448-461.
`
`* cited by examiner
`
`Primary Examiner—Edward F. Urban
`Assistant Examiner—Sheila Smith
`(74) Attorney, Agent, or Firm—Pillsbury Madison & Sutro
`
`(57)
`
`ABSTRACT
`
`(87) PCT Pub. No.: WO97/39599
`
`PCT Pub. Date: Oct. 23, 1997
`
`The present invention relates to a handover method in a
`mobile communication system having a multilayer radio
`coverage, the system comprising at least one microcell (1,2,
`3) and at least one macrocell (M). The coverage area of the
`microcell (1, 2, 3) is at least mainly within the coverage area
`of the macrocell (M), the microcell layer forming a lower
`cell layer and the macrocell layer forming a highercell layer.
`Int. C1? one H04Q 7/20; GO1S 13/58
`(S51)
`Acell to be examined andatarget cell are located at different
`(52) U.S. C0. cee eceseeeeeeeee 455/444; 455/441; 455/453;
`cell layers in the mobile communication system. In the
`342/109
`method, relative moving speeds of mobile stations (MS) in
`(58) Field of Search 0... 455/437, 441,
`the service area of a cell at the lower cell layer are deter-
`455/444, 453, 449, 525; 342/109, 107;
`mined anda traffic load in a cell at the higher cell layer is
`370/331
`monitored. The handover method of the invention is char-
`acterized in that in the method the service times are deter-
`mined in the service area of the cell at the lowercell layer,
`a percentage P of the numberof the service timesrestricted
`by a specific pre-set threshold service time R in a selected
`5,262,263 * 11/1993 Ohkawaet al. oo. 430/66
`service time material is determined, a percentage ofall the
`5,396,645 * 3/1995Huffoo.cecssssseeesseeees 455/33.4
`
`
`mobile stations (MS) within the service area of the cell to be
`5,513,380 * 4/1996Ivanovetal.vveseeseeccssssssseeees 455/33.2
`
`
`examined is handed overto the target cell in the order of the
`5,678,185 * 10/1997Chiao...ccesesssssssssseeeessseeeees 455/33.2
`
`relative moving speed,and the pre-set threshold service time
`5,722,072 *
`2/1998
`fo
`seeeeseeseseeeseneeee 455/437
`R is adjusted to attain a desired traffic load level in the cell
`5,913,168 *
`6/1999 Moreau et al. we 455/441
`at the uppercell layer.
`FOREIGN PATENT DOCUMENTS
`
`(30)
`
`Foreign Application Priority Data
`
`Apt. 16, 1996
`
`(FD)
`
`ssssssssssssssssssssseessessssssssssesssssssstenseees 961668
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`526 436
`
`2/1993 (EP).
`
`11 Claims, 2 Drawing Sheets
`
`START
`
`
`
`Calculate P%
`service time < R
`
`33
`
`P% of the fastest MS's
`to the macro cell
`
`
`
`Increase
`Macrocell load
`value R
`<LopT?
`
`
`
` ——
`
`
`> LMax?
`
`
`
`
`Decrease
`value R
`
`EXHIBIT 1013
`
`Macrocell load
`
`1
`
`EXHIBIT 1013
`
`

`

`U.S. Patent
`
`Mar.27, 2001
`
`Sheet 1of 2
`
`US 6,208,863 BI
`
`GMSC
`
`PSTN
`PSPDN
`ISDN
`
`OMC
`
`BSC
`
`=
`
`<a
`
`annyYY<iaa
`
`oo00
`
`2
`
`
`
`

`

`U.S. Patent
`
`Mar. 27,2001
`
`Sheet 2 of 2
`
`US 6,208,863 B1
`
`Fig. 3
`
`,
`
`O2
`
`a
`
`3d
`
`Calculate P%:
`service time <R
`
`PY of the fastest MS's
`to the macro cell
`
`34
`
`Yes
`
`increase
`value R
`
`3D
`
`Macrocell load
`<LopT?
`
`
`No
`
`JO
`
`
`
`Macrocell toad
`> Lyax ?
`
`
`Yes
`
`BT
`
`-
`
`Decrease
`value R
`
`
`
`3
`
`

`

`US 6,208,863 B1
`
`1
`HANDOVER IN A MOBILE
`COMMUNICATION SYSTEM HAVING A
`MULTILAYER RADIO COVERAGE
`
`FIELD OF THE INVENTION
`
`2
`FIG. 2 shows a simplified block diagram of the pan-
`European GSM mobile communication system. The units
`shown in FIG. 2 are present also in other mobile commu-
`nication networks but they may have different names. A
`mobile station MS is connected over a radio path to one of
`base transceiver stations (BTS), such as BTS1 in FIG. 2. The
`The present invention relates to a handover method in a
`mobile station MS constantly measures signals of the adja-
`mobile communication system of a multilayer radio cover-
`cent base stations for a possible handover. A base station
`age comprising a lower cell
`layer having at
`least one
`system BSS comprises a base station controller BSC and
`microcell and a higher cell
`layer having at
`least one
`base stations BTS under its control. Several base station
`macrocell, the microcell being at least mainly within the
`controllers BSC generally operate under a mobile services
`macrocell, the method comprising the steps of determining
`switching centre MSC. The mobile services switching centre
`relative moving speeds of mobile stationsinafirst cell at the
`MSC is connected to other mobile services switching centres
`lowercell layer, and monitoringa traffic load in a second cell
`and via a gateway mobile services switching centre (GMSC)
`at the higher cell layer.
`to a public switched telephone network. The operation of the
`The invention also relates to a handover method from a
`whole system is supervised by an operation and maintenance
`centre OMC. Subscriber data of the mobile station MS is
`microcell to a macrocell in a mobile communication system
`having a multilayer cell coverage comprising at least one
`permanently stored in a home location register HLR and
`microcell and at least one macrocell, whereby the microcell
`temporarily to a visitor location register VLR on whosearea
`the mobile station MSis located at each time.
`is within the macrocell,
`the method comprising steps of
`determining relative moving speeds of mobilestations in the
`WO95/28813 discloses a cellular mobile communication
`service area of the microcell, and monitoringa traffic load in
`system where the time the mobile station is registered with
`the macrocell.
`a microcell is measured and the direction of travel of Othe
`mobile station is defined. A mobile station is indicated as
`being fast if the measured time in a cell is shorter than the
`pre-set time limit of this microcell and the entrance direction
`of the mobile station to the respective cell is different from
`the exit direction. In the prior art system, fast moving mobile
`stations are handed over to use a macrocell network. Han-
`dover from a microcell to a macrocell is carried out only if
`the mobile station has been found to be moving fast in
`several microcells. The slow speed of the handover decision
`is a problem with this solution. The mobile station has to
`pass through several microcells until a decision to carry out
`a handoverto a macrocell can be made. Duringthis time, the
`network is loaded with too frequent handovers from one
`microcell to another. A further problem is a possible block-
`ing of a macrocell when all the mobile stations interpreted
`as being fast are handed over to the macrocell.
`European Patent Application 0 589 278 discloses a
`microcell/macrocell system. This prior art system involves
`measuring the time a mobile station moving in a macrocell
`remains within a microcell inside the macrocell. The prior
`art system prevents fast moving mobile stations from being
`handed over to a base station of the microcell. Handover
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`from the macrocell to the microcell is carried out only if the
`mobile station is still within the microcell after a predeter-
`mined time after its arrival in the microcell. If the mobile
`station has already departed from the microcell during the
`predetermined time, handover from the macrocell to the
`microcell will not be carried out. The slow speed of the
`handover decision is a problem with this solution,
`too.
`Handover decision can be made only after a predetermined
`time period after the mobile station has arrived in the
`microcell. After this time has passed, the mobile stations in
`the area of the microcell are interpreted as being slow and
`are handed over to a microcell although the mobile station
`would be just about to depart from the microcell. During the
`waiting time of the handover decision, the mobile station
`loads the macrocell. A problem with this solution is also an
`erroneous conception of the speed of a mobile station. The
`handoveris affected only by the arrival of the mobile station
`in the microcell and by the location of the mobile station
`after a predetermined time from thearrival. Duringthis time,
`a fast moving mobile station may have departed from the
`microcell and returned there,
`in which case the system
`carries out a handover from the macrocell to the microcell.
`
`50
`
`55
`
`60
`
`65
`
`BACKGROUND OF THE INVENTION
`
`A radio coverage area in mobile communication systems
`is implemented with different cell sizes and often even with
`multilayer radio coverages of base stations. A cell having a
`very small coverage area,
`i.e. a microcell,
`is used for
`example in areas which are difficult to cover or require a
`high subscriber capacity. A larger coverage area is imple-
`mented with a macrocell, which often encompasses the
`geographical area of one or more microcells. In a multilayer
`radio coverage implemented in this way, macrocells repre-
`sent coverage areas of a higher layer and microcells those of
`a lowerlayer. The mobile station may move from onecell to
`another and register with a new base station. When the
`mobile station has a call in progress, switching the call to the
`new cell is also involved, in which case switching to a new
`base station is referred to as handover.
`
`FIG. 1 of the accompanying drawing discloses an
`example of a two-layer radio coverage in a mobile commu-
`nication system. The simplified cell areas in FIG. 1 illustrate
`the areas where the base station of each cell can offer a
`
`sufficient signal level to the mobilestation. In FIG. 1,a large
`macrocell M encompassesthe areas of smaller microcells 1,
`2 and 3. Amobile station MS located in position x within the
`microcell 1, for example, can in principle be served by the
`base station of either the macrocell M or the microcell 1.
`
`In radio networks implemented with a multilayer radio
`coverage,it is advantageousto utilize the possibilities pro-
`vided by the different cell layers by commanding the mobile
`stations, on the basis of someclassification, to register with
`the most suitable base station, while also avoiding unnec-
`essary handovers. One such classification principle is the
`speed of a mobile station in a mobile communication
`network having one or more microcells whose coverage
`areas are entirely inside anothercell, typically a macrocell.
`It is known in the art that slow moving mobile stations are
`advantageously handed over to a base station of some
`microcell to prevent them from unnecessarily load the base
`station of the macrocell. On the other hand, it is advanta-
`geous to hand fast moving mobile stations over to the base
`station of the macrocell
`to prevent frequently recurring
`handovers and even to prevent calls from being lost when
`mobile stations move fast beyond the coverage area of the
`microcell.
`
`4
`
`

`

`US 6,208,863 B1
`
`3
`In one embodimentof the system, the measurementof time
`is stopped if the mobile station departs from the microcell,
`and it is started again from the beginning when the mobile
`station returns to the microcell area. The timer will be reset,
`for example, if the mobile station moves beyond the cov-
`erage area of a microcell momentarily,or if the measurement
`result is erroneous/missing, in which case the mobile station
`will not be recognized as being slow.
`BRIEF DESCRIPTION OF THE INVENTION
`
`An object of the present invention is to implementa fast
`and flexible handover decision for transferring a mobile
`station from one cell layer to another in a mobile commu-
`nication system having a multilayer radio coverage.
`This will be attained by a handover method, which is
`according to the invention characterized by further steps of
`determining the service timesin the first cell at the lowercell
`layer, determining a percentage P of the number of the
`service times confined by a specific pre-set threshold service
`time R amonga selected service time material, handing over
`the percentage P ofall the mobile stations within the service
`area of the cell to be examinedto the target cell in the order
`of the relative moving speed of the mobile stations, the cell
`to be examined beingthefirst cell and the target cell being
`the second cell, or vice versa, and adjusting the pre-set
`threshold service time R to attain a desired traffic load level
`
`10
`
`15
`
`20
`
`25
`
`in the second cell at the upper cell layer.
`The invention is based on the idea that a specific portion
`of all the mobile stations within the service area of the cell
`
`4
`area of a microcell at a lower cell layer are defined. The
`percentage P of the service times confined by a specific
`pre-set
`threshold service time R is calculated from the
`sample of the service times. The calculated percentage
`numberof all the mobile stations currently present within
`the service area of the cell to be examined will be handed
`over to the target cell in the order of the relative moving
`speed of the mobile stations. The load in a cell at a higher
`cell layer is monitored continuously. When load deviates
`from the desired load level, the pre-set threshold service
`time R is changed so that the number of mobile stations to
`be handed overto the target cell will change into the desired
`direction.
`In the handover method of the invention from a microcell
`to a macrocell,
`the relative speeds and service times of
`mobile stations in the service area of the microcell are
`defined. The percentage P of service times shorter than a
`specific pre-set threshold service time R is calculated from
`a sample of the service times. The calculated percentage P
`of all the mobile currently present within the service area of
`the microcell will be handed over to a macrocell in the order
`of the relative moving speed of the mobilestations, starting
`form the fastest mobile station. The load in the microcell is
`monitored. When load exceeds the maximum loadlevel, the
`pre-set threshold service time R is decreased. When the load
`is smaller than the desired load level, the threshold service
`time R is appropriately increased.
`
`BRIEF DESCRIPTION OF THE FIGURES
`to be examined is handed overtoatarget cell at another cell
`layer on the basis of the moving speed of the mobile station.
`In the following, the invention will be described in greater
`The number of the mobile stations to be handed over is
`detail with reference to the accompanying drawings,
`in
`which
`determined by a long-term statistics on the time the mobile
`subscribers stay in a cell at a lowercell layer, i.e. a service
`time.
`
`The advantage of the inventive handover from one cell
`layer to anotheris that a handoverdecision can be madefast.
`A further advantage of the handover method of the
`invention is that an optimal utilization of the cell layer is
`attained in a flexible and effective manner and unnecessary
`handovers are avoided. The utilization of the cell layers can
`be adjusted easily and consistently. The handover method of
`the invention will adapt
`to changing traffic levels and
`varying speed distributions of mobile stations.
`Another advantage of the invention is a simple and
`reliable handovercriterion which is suitable for various cell
`
`sizes and also for networks having varying cell sizes. The
`use of thelast statistical data on the service times in decision
`
`making increases the stability of the solution.
`The inventionalso relates to a handover method as shown
`in the preamble from a microcell to a macrocell, which is
`characterized by further steps of determining the service
`times in the microcell, calculating a percentage P of the
`number of the service times shorter than a specific pre-set
`threshold service time R among a selected service time
`material, handing over the percentage P of all the mobile
`stations within the service area of the microcell
`to the
`macrocell in the order of the relative moving speed,starting
`from the fastest mobile station, and adjusting the pre-set
`threshold service time R to attain a desired traffic load level
`in the macrocell.
`
`In addition to the advantages of the above handover
`method, an advantage of the handover method from a
`microcell to a macrocell is a handovercriterion value which
`is easily adjustable and commonto several cells.
`In the handover method of the invention,
`the relative
`speeds and service times of mobile stations in the service
`
`35
`
`FIG. 1 illustrates a two-layer radio coverage of a mobile
`communication system,
`FIG. 2 is a block diagram of the structure of a mobile
`communication system,
`FIG. 3 is a flow chart of a preferred embodiment of the
`method of the invention, and
`FIG. 4 shows an example of service time distribution in
`a cell.
`
`45
`
`50
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`invention can be applied in any cellular
`The present
`mobile communication system. In the following, the inven-
`tion will be described in greater detail with reference, by
`way of example, to the pan-European digital mobile com-
`munication system GSM. FIG. 2 illustrates the above-
`described structure of a GSM network in a simplified
`manner. For a more accurate description of the GSM system,
`reference is made to the GSM recommendations and “The
`
`55
`
`GSM System for Mobile Communications” by M. Mouly
`and M- B. Pautet, Palaiseau, France, 1992,
`ISBN:
`2-9507190-0-7.
`
`60
`
`The method of the invention is particularly applicable to
`microcell/-macrocell networks. FIG. 1 shows an example of
`a microcell/macrocell network having a two-layer radio
`coverage. The method is also suitable for use in other
`networks implemented with multilayer cell coverages, 1.e.
`with more than twocell layers.
`The handover method of the invention is most advanta-
`
`65
`
`geous when mobile stations are handed over from the
`microcell layer to the macrocell layer, in which case the cell
`to be examined in the method is a microcell at a lower cell
`
`layer and the target cell is a macrocell at the highercell layer.
`
`5
`
`5
`
`

`

`US 6,208,863 B1
`
`5
`In the following, the invention will be explained in more
`detail primarily in the light of this preferred embodiment.
`FIG. 3 is a flow chart of a preferred embodiment of the
`method of the invention. In the method, the time a mobile
`station is located in the service area of microcell 1, 2 or 3 is
`measured for each mobile station MS. This service time will
`be determined on the basis of two consecutive handovers of
`
`the mobile station MS, for example. By the first handover,
`the mobile station MS enters the microcell 1 and by the
`second handover departs from the microcell 1 and will be
`served by the following cell. The handover criterion
`employed in these handovers is other than the handover
`criterion in the method ofthe invention. If the mobile station
`is handed over to the macrocell M on the basis of the
`
`handovercriteria of the method of the present invention, the
`service time will be conceived as the time the mobile station
`MSwould have been switched to the microcell 1 if handover
`to the macrocell M had not beencarried out. In this case the
`
`service time will thus be determined by the time between the
`first handover of the mobile station MS to the microcell 1
`and the mobile station MS departing from the service area of
`the microcell 1. Departure from the service area of the
`microcell is determined, for example, on the basis of normal
`base station signal measurements in the network. The mea-
`sured service times are stored microcell-specifically into a
`continuously storing ring register. The service times mea-
`sured during a specific time period or a specific numberof
`last measured service times can be read from the ring
`register.
`The criterion for a handover decision in the method is a
`pre-set threshold service time R (step 31 in FIG. 3). The
`threshold service time R is macrocell-specific and thus the
`same for all different sized microcells 1, 2 and 3 located
`within the area of the macrocell M. The percentage P of the
`service times which are shorter than the threshold service
`time R is calculated from the stored service times in the area
`of the microcell 1 (step 32). The calculation is carried out on
`a selected sample of service times. The selected sample of
`the service times may include the service times measured
`and stored during thelast half-hour or the last stored hundred
`service time values. The length of the time period covered by
`the sample of the service times or the number of service
`times in the sample are selected to be appropriate for the
`purpose. The value P derived as the result of calculation
`indicates how many per cents of the mobile stations MS
`would have been served by the base station BTS of the
`microcell 1 for a shorter time than the threshold service time
`
`R during the last half-hour, for example, without the han-
`dover of the present invention. In step 33 the fastest mobile
`stations of the mobile stations currently engaged with the
`base station BTS of the microcell 1 are handed over one by
`one from the microcell 1 to the macrocell M. As a result, P%
`of all the mobile stations which are engaged or have been
`engaged with the base station BTS of the microcell 1 before
`the above-described handover of the invention will be
`engaged with the base station BTS of the macrocell M after
`the inventive handover. If the fastest P% of the mobile
`stations having been served by the microcell (still within the
`service area of the microcell 1) have already been handed
`over to the macrocell M, no new handovers of mobile
`stations will be carried out. The relative moving speed of the
`mobile stations MSin the service area of the microcell 1 is
`determined in any appropriate manner. Only ranking the
`mobile stations into an order of their speed is essential for
`determining the relative speed. It is not significant for the
`invention how this ranking of mobile stations according to
`their speed is attained. Some methods for determining the
`
`6
`speed of the mobile station MS have been disclosed e.g. in
`WO 92/01950 and WO 97/00587.
`Thetraffic load in the macrocell M is monitored in order
`
`that the network would not be blocked by handovers. In step
`34 of FIG. 3, the load in the macrocell M is compared with
`a pre-set optimum load L,,,. If the load is smaller than the
`optimum L,,,. the value of the macrocell-specific threshold
`service time R is increased (step 35) so that a greater portion
`of the load in the microcells 1, 2 and 3 would be directed to
`the macrocell M. In step 36 the traffic load in the macrocell
`M is compared with a pre-set maximum load valueL,,,,. If
`the maximum load L,,,,.
`is exceeded,
`the value of the
`threshold service time R is decreased to diminish the number
`of the mobile stations MSto be handed over(step 37). In the
`subsequent calculation of service times for the microcells 1,
`2 and 3, the new threshold service time R will be used in
`place of the previously set threshold service time R. By
`adjusting one of the criterion parameters of the handover,
`here the threshold service time R, the threshold for handover
`from onecell layer to another can be commonly adjusted for
`several microcells 1, 2 and 3. Whenthe load values L,, and
`Lae are equal, a continuous adjustment of the threshold
`service time R is carried out in the inventive method. Some
`microcells can, when required, be assigned a dedicated
`microcell-specific threshold service time, by means of which
`the 30 macrocell-specific threshold service time R can be
`fine-adjusted.
`FIG. 4 shows one service time distribution by way of
`example.
`The horizontal axis of FIG. 4 represents the service time
`T and the vertical axis the probability density f(T). The
`threshold service time R is indicated by a vertical line in
`FIG. 4. The portion of the distribution graph shown on the
`left side of the vertical line R represents the proportion of the
`mobile stations whose service time is shorter than the
`threshold value R.
`
`In mobile communication systems having more than two
`cell layers, the method can be applied by selecting the target
`cell from any oneof the highercell layers. All cell layers can
`be covered by chaining the use of the method in a desired
`manner.
`
`Whenthe method of the invention is applied to handover
`from a macrocell to a microcell, the principles are as in the
`above described handover from a microcell to a macrocell.
`
`When mobile stations are to be handed over from a higher
`cell layer to a lower cell layer, microcell-specific service
`times are measured and the load in the macrocell M is
`
`monitored as described above. The microcell-specific per-
`centage P,, of the service times longer than the macrocell-
`specific threshold value R is calculated from microcell-
`specific service time distributions. The percentage P, of the
`mobile stations MS located within the service area of the
`
`microcell 1 but engaged with the macrocell M is handed
`over to the microcell 1, starting from the slowest mobile
`stations. When the load in the macrocell falls below the
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`the threshold service time R is
`desired load level L,,,,
`increased and when the load exceeds the maximum load
`
`L,,a» the threshold service time R is decreased. If the
`maximum traffic load in the microcell 1 was exceeded as a
`
`result of the handover, mobile stations are not handed from
`the macrocell M overto the microcell 1. With the handover
`
`65
`
`method of the present invention, more mobile stations are
`handed over to microcells having larger service areas, for
`
`6
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`US 6,208,863 B1
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`example to the microcell 2, than to smaller microcells 1 and
`3, although the threshold service time R is commonto all
`microcells 1, 2 and 3 in the area of the macrocell M.
`The calculation and handoversin the method mayalso be
`carried out at specific intervals, for example. In the same
`microcell/macrocell environment the method of the inven-
`tion can be applied to both directions, that is, for a handover
`from a microcell to a macrocell and for a handover from a
`macrocell to a microcell. In this case faster mobile stations
`
`are handed over to the macrocell and, correspondingly,
`slower mobile stations to the microcell.
`
`the method of the
`The arrangement for carrying out
`invention is preferably embodied in association with han-
`dover decision making equipments, for example, in asso-
`ciation with a base station controller BSC. The arrangement
`can also be distributed to different units of the system, for
`example by placing the means defining the service times in
`the base station BTS of the cell
`to be examined. The
`
`arrangement comprises registration means, timing means,
`calculation means and a handover control unit. Each cell is
`
`provided with dedicated means.
`The drawings and the associated description are intended
`merely to illustrate the inventive concept. In its details the
`handover method of the invention in a mobile communica-
`
`tion system of a multilayer radio coverage may be modified
`within the scope of the claims. Although the invention has
`been explained above primarily in association with a han-
`dover carried out from a microcell
`to a macrocell,
`the
`method is also applicable to handovers carried out in the
`opposite direction. The method is very well suited to be used
`even in CDMA mobile communication systems where cell
`sizes in the network are changed on the basis of traffic
`capacity.
`Whatis claimedis:
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`measuring the service times in the service area ofthe first
`cell at the lower cell layer, and
`
`storing the service times into a continuously storing
`register.
`3. A method according to claim 1, characterized in that
`said service time material comprises the service times deter-
`mined during a specific time period.
`4. A method according to claim 1, characterized in that
`said service time material comprises a specific numberof the
`last determined service times.
`
`5. A method according to claim 1, characterized in that
`whenthe target cell is the second cell at the higher cell layer
`and the cell to be examinedisthefirst cell at the lowercell
`
`layer,
`
`said service times confined by the pre-set threshold ser-
`vice time R comprise the service times that are shorter
`than the threshold service time R, and
`
`the percentage P of the mobile stations (MS) to be handed
`over to the target cell comprise the fastest mobile
`stations (MS).
`6. A method according to claim 1, characterized in that
`whenthe target cell is the first cell at the lower cell layer and
`the cell to be examined is the second cell at the highercell
`layer,
`
`said service times confined by the pre-set threshold ser-
`vice time R comprise the service times that are longer
`than the threshold service time R,
`
`the percentage P of the mobile stations (MS) to be handed
`over to the target cell comprise the slowest mobile
`stations (MS), and
`
`1. Ahandover method in a mobile communication system
`of a multilayer radio coverage comprising a lowercell layer
`having at least one microcell (1, 2, 3) and a higher cell layer
`having at least one macrocell (M), the microcell (1, 2, 3)
`being at least mainly within the macrocell (M), the method
`comprising the steps of:
`determining relative moving speeds of mobile stations
`(MS)in a first cell at the lower cell layer, and
`monitoring a traffic load in a second cell at the highercell
`layer, characterized by further steps of:
`determining the service times in thefirst cell at the lower
`cell layer,
`determining a percentage P of the numberof the service
`times confined by a specific pre-set threshold service
`characterized by further steps of:
`time R amongaselected service time material,
`55
`determining the service times in the microcell (1, 2, 3),
`handing over the percentage P of all the mobile stations
`(MS) within the service area ofthe cell to be examined
`to the target cell in the order of the relative moving
`speed of the mobile stations, the cell to be examined
`being the first cell and the target cell being the second
`cell, or vice versa, and
`adjusting the pre-set threshold service time R to attain a
`desiredtraffic load level in the second cell at the upper
`cell layer.
`2. A method according to claim 1, characterized in that
`said step of determining the service timesofthe first cell at
`the lower cell layer comprises the following steps:
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`the mobile stations (MS)are not handed overto the target
`cell if the pre-set maximum valueofthetraffic load in
`the target cell is exceeded.
`7. Ahandover method from a microcell to a macrocell in
`
`a mobile communication system having a multilayer cell
`coverage comprising at least one microcell (1, 2, 3) and at
`least one macrocell (M), whereby the microcell (1, 2, 3) is
`within the macrocell (M), the method comprising steps of:
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`determining relative moving speeds of mobile stations
`(MS) in the service area of the microcell, and
`
`monitoring a traffic load in the macrocell (M),
`
`calculating a percentage P of the number of the service
`times shorter than a specific pre-set threshold service
`time R among a selected service time material,
`
`handing over the percentage P of all the mobile stations
`(MS) within the service area of the microcell (1, 2, 3)
`to the macrocell (M) in the orderof the relative moving
`speed, starting from the fastest mobile station (MS),
`and
`
`adjusting the pre-set threshold service time R to attain a
`desired traffic load level in the macrocell (M).
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`US 6,208,863 B1
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`9
`8. A method according to claim 7, characterized in that
`said step of determining the service times of the microcell
`(1, 2, 3) comprises the following steps:
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`10. A method according to claim 7, characterized in that
`said service time material comprises a pre-set numberof the
`last determined service times.
`
`measuring the service times in the service area of the
`microcell (1, 2, 3), and
`storing the service times into a continuously storing
`register.
`
`9. A method according to claim 7, characterized in that
`said service time material comprises the service times deter-
`mined during a pre-set time period.
`
`11. A method according to claim 7, characterized in that
`said adjustment of the pre-set
`threshold service time R
`comprises
`increasing the threshold service time R whenthe load in
`the macrocell (M) is to be increased, and
`decreasing the threshold service time R when the load in
`the macrocell (M) is to be decreased.
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