(12) United States Patent
`Cuffaro et al.
`
`111111
`
`1111111111111111111111111111111111111111111111111111111111111
`US006418317Bl
`US 6,418,317 B1
`Jul. 9, 2002
`
`(10) Patent No.:
`(45) Date of Patent:
`
`(54) METHOD AND SYSTEM FOR MANAGING
`FREQUENCIES ALLOCATED TO A BASE
`STATION
`
`(75)
`
`Inventors: Angelo Cuffaro, Pierrefonds; Michel
`Desgagne, St-Hubert, both of (CA);
`Arne Simonsson, Gammelstad (SE)
`
`(73) Assignee: Telefonaktiebolaget LM Ericsson
`(publ), Stockholm (SE)
`
`( *) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.: 09/452,566
`
`(22) Filed:
`
`Dec. 1, 1999
`
`(51)
`(52)
`
`(58)
`
`(56)
`
`Int. Cl? .................................................. H04Q 7/20
`U.S. Cl. ......................... 455/450; 455/62; 455/452;
`455/509
`Field of Search ................................. 455/446, 450,
`455/451, 452, 453, 447, 509, 62
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`5,280,630 A * 1!1994
`5,490,137 A * 2/1996
`5,666,655 A * 9/1997
`5,701,590 A
`12/1997
`5,737,705 A * 4/1998
`5,898,928 A * 4/1999
`6,295,453 B1 * 9/2001
`
`Wang ......................... 455!450
`Hulsebosch et a!. ........ 455!450
`Ishikawa et a!. ............ 455!450
`Fujinami ..................... 455!62
`Ruppel et a!.
`.............. 455!452
`Karlsson et a!. ............ 455!450
`Desgagne et a!.
`.......... 455!450
`
`~18
`~
`
`FOREIGN PATENT DOCUMENTS
`
`EP
`EP
`wo
`wo
`wo
`wo
`wo
`wo
`
`6/1997
`0 817 521 A2
`1!2000
`1022921
`wo 9713388
`4/1997
`wo 9732444
`9/1997
`wo 98/48586
`4/1998
`wo 98/56204
`6/1998
`wo 9905878
`2/1999
`wo 0060896
`10/2000
`OTHER PUBLICATIONS
`International Search Report, PCT/SE00/02270, dated May
`28, 2001.
`* cited by examiner
`Primary Examiner-William Trost
`Assistant Examiner-Simon Nguyen
`(74) Attorney, Agent, or Firm-Jenkens & Gilchrist; Sandra
`Beauchesne
`(57)
`
`ABSTRACT
`
`The present invention is a method and system for managing
`frequencies allocated to a cell within a cellular network to
`assign certain ones of those allocated frequencies for use by
`channel equipment within that cell. The method measures at
`least one quality metric for each of the allocated frequencies.
`At least one measured quality metric for the unassigned
`frequencies are compared against at least one quality metric
`for the assigned frequencies. An unassigned frequency is
`swapped for an assigned frequency based upon the com(cid:173)
`parison step. Additionally, a voting step is used to indicate
`that either the unassigned frequency or the assigned fre(cid:173)
`quency has a higher signal quality for communication.
`
`34 Claims, 6 Drawing Sheets
`
`/
`12(5)
`
`0001
`
`Marvell Semiconductor, Inc.
`MediaTek Inc.
`MediaTek USA, Inc.
`Exh. 1004
`IPR of U.S. Pat. No. 7,477,624
`
`

`

`U.S. Patent
`
`Jul. 9, 2002
`
`Sheet 1 of 6
`
`US 6,418,317 B1
`
`18
`
`12(3)
`(
`
`/
`12(5)
`
`14
`/ 22
`I IRF
`r-... 34
`LVM
`
`30
`-
`
`BASE STATION
`
`34
`
`32 --
`I
`
`22
`
`IIRF
`res
`Tx/Rx
`20(1)
`-
`
`22
`
`II RF
`IIRF
`res
`ICS / \
`Tx/Rx
`Tx/Rx
`20(N)
`20(2)
`-
`
`t---
`I'--
`
`34
`32
`
`PROCESSOR
`
`FIG. 2
`
`0002
`
`

`

`U.S. Patent
`
`Jul. 9, 2002
`
`Sheet 2 of 6
`
`US 6,418,317 B1
`
`START
`
`300
`
`MAKE
`MEASUREMENTS
`
`COMPARE
`MEASUREMENTS
`
`305
`
`31 0
`
`YES
`
`SW/JP ASSIGNED
`AND UNASSIGNED
`FREQUENCIES
`(PERFORM FREQUENCY
`PACKING IF NECESSARY)
`
`330
`
`DONE
`
`335
`
`FIG. 3
`
`0003
`
`

`

`U.S. Patent
`
`Jul. 9, 2002
`
`Sheet 3 of 6
`
`US 6,418,317 B1
`
`/ 400
`FREQUENCY
`
`405
`/
`INTERFERENCE
`(dBm)
`
`410
`/
`USE
`
`/ 415
`ASSIGNED
`
`F1
`
`F8
`
`F15
`
`F22
`
`F29
`
`F36
`
`F43
`
`F50
`
`USED
`
`USED
`
`IDLE
`
`IDLE
`
`ASSIGNED
`
`ASSIGNED
`
`ASSIGNED
`
`ASSIGNED
`
`UNASSIGNED
`
`UNASSIGNED
`
`UNASSIGNED
`
`UNASSIGNED
`
`-117
`
`-106
`
`-110
`
`-105
`
`-118
`
`-95
`
`FIG. 4
`
`0004
`
`

`

`U.S. Patent
`
`Jul. 9, 2002
`
`Sheet 4 of 6
`
`US 6,418,317 B1
`
`s
`
`F29
`
`D
`
`F36
`
`F43
`
`F50
`
`t
`
`500
`
`/usm 1 51o
`I
`
`IDLE
`
`Fl
`
`F8
`
`F15
`
`F22
`
`505
`
`-1
`
`-1
`
`-1
`
`-1
`
`+1
`
`-1
`
`+1
`
`-1
`
`Note: +3d8 hyster esis
`added to each unassigned
`interference measurement
`before voting
`
`FIG. SA
`
`515
`
`515
`
`cs: F1
`
`F29
`
`+2
`
`D
`
`F36
`
`F43
`
`F50
`
`-8
`
`+6
`
`-8
`
`t
`500 FIG. 5B
`
`F8
`
`F15
`
`F22
`
`505
`
`-6
`
`-8
`
`+2
`
`-6
`
`+6
`
`+8
`
`-4
`
`-6
`
`+6
`
`-8
`
`-6
`
`-4
`
`0005
`
`

`

`U.S. Patent
`
`Jul. 9, 2002
`
`Sheet 5 of 6
`
`US 6,418,317 B1
`
`ASSIGNED
`
`UNASSIGNED
`F29
`
`F36
`
`F15
`
`F50
`
`515
`
`F1
`
`F8
`
`F43
`
`F22
`
`+2
`
`-8
`
`-6
`
`-8
`
`-8
`
`-8
`
`-8 /
`
`-4
`
`-6
`
`520
`
`v--~
`' r-- 525
`-4
`
`500 FIG. 5C
`
`ASSIGNED
`
`UNASSIGNED
`F29
`
`F36
`
`F15
`
`F50
`
`F1
`
`F8
`
`F43
`
`F22
`
`+2
`
`-8
`
`-4
`
`-8
`
`-6
`
`-4
`
`-8
`
`-2
`
`-4
`
`-6
`
`-2
`
`-8
`
`-2
`
`-8
`
`0
`
`-4
`
`FIG. 5D
`
`0006
`
`

`

`U.S. Patent
`
`Jul. 9, 2002
`
`Sheet 6 of 6
`
`US 6,418,317 B1
`
`:s: F1
`
`F8
`
`D
`
`F29
`
`( +30,+ 12) (+28,+14)
`
`F36
`
`(+28,+14) ( +30,+ 12)
`
`F43
`
`( -30,-30) ( -1 0,-30)
`
`/
`
`Fso/ (+30,+10) (-8,-10)
`
`I
`(x,y)=(uphnk differential,downlink differential)
`600
`FIG. 6A
`
`~ F36
`
`F29
`
`D
`
`F8
`
`( -30,-12) ( -28,-14)
`
`( -28,-14) ( -30,-12)
`
`F1
`
`F43
`
`F50
`
`FIG. 6B
`
`0007
`
`

`

`US 6,418,317 B1
`
`2
`One problem of the previously described telecommuni(cid:173)
`cations systems implementing interference driven channel
`selection is a degradation of signal quality when increasing
`capacity on the telecommunications system. Another prob-
`lem is that the gain of current channel selection methods is
`decreased during periods of high traffic load. Without unas(cid:173)
`signed frequencies, there are fewer free frequencies to select
`among when assigning calls to traffic channels and the gain
`degrades using interference reducing methods, such as qual-
`10 ity driven channel selection (QDCS) and interference driven
`channel selection (IDCS).
`
`5
`
`1
`METHOD AND SYSTEM FOR MANAGING
`FREQUENCIES ALLOCATED TO A BASE
`STATION
`BACKGROUND OF THE INVENTION
`1. Technical Field of the Invention
`The present invention relates to cellular telephone
`systems, and, in particular, to a method for managing the use
`of frequencies allocated to a cell.
`2. Description of Related Art
`Cellular telephone systems divide a large service area into
`a number of smaller discrete geographical areas called
`"cells" each typically ranging in size from about one-half to
`about twenty kilometers in diameter. Each cell is contiguous
`with multiple adjacent cells to provide continuous coverage
`throughout the service area. A base station including a 15
`plurality of transceivers (i.e., channel equipment) capable of
`operating independently on different radio frequencies is
`provided for each of the cells. Via the transceivers, the base
`stations engage in simultaneous communications with plural
`mobile stations operating within the area of the associated 20
`cell. The base stations further communicate via data links
`(and voice trunks) with a central control station, commonly
`referred to as a mobile switching center, which functions to
`selectively connect telephone calls to and from the mobile
`stations through the base stations and, in general, control 25
`operation of the system.
`Each cell is allocated the use of a predetermined set of
`frequencies, wherein each frequency comprises a physical
`channel supporting a plurality of logical channels (i.e.,
`timeslots) therein. The availability of multiple frequencies 30
`per cell, with multiple logical channels per frequency, per(cid:173)
`mits base stations to simultaneously handle many telephone
`conversations with many mobile stations. The frequencies
`allocated to a cell are preferably spaced apart across the
`frequency spectrum of the cellular band such that adjacent 35
`cells are not assigned close frequencies. This serves to
`minimize the instances of adjacent channel interference.
`Because only a limited number of frequencies are avail(cid:173)
`able in the cellular band, an allocation of the same frequen(cid:173)
`cies is repeated (i.e., reused) in other cells in a distant part 40
`of large service areas with many cells. No adjacent cells,
`however, are allocated the same frequency. Furthermore, the
`power levels of the signal transmissions on any given
`frequency are limited in strength. The foregoing precautions
`serve to minimize the likelihood of co-channel interference
`caused by reuse of that same frequency in a distant cell.
`Although each cell is allocated certain specific frequen(cid:173)
`cies and those specific frequencies are reused in a distant
`part of a large service area, it has been shown that interfer(cid:173)
`ence may increase to the point of degrading quality when the 50
`frequency reuse plan is changed from, for instance, a 7/21
`reuse to a 4/12 reuse, without over-dimensioning the trans(cid:173)
`ceivers. Traditional channel selection techniques for select(cid:173)
`ing the traffic channels for mobile stations use, such as
`interference driven channel selection (IDCS), are unable to 55
`overcome the quality degradation of reduced frequency
`reuse plan cellular systems. In order to improve the quality,
`the idea of creating virtual frequencies has developed to
`enable an increase in system capacity. The set of frequencies
`that are allocated to the cell are often referred to as "virtual 60
`frequencies". The virtual frequency set includes assigned
`and unassigned frequencies. Assigned frequencies are those
`frequencies operating on available transceivers with a base
`station serving a cell. Unassigned frequencies are created by
`allocating more frequencies to a base station serving a cell
`than there are available transceivers to handle those frequen(cid:173)
`Cies.
`
`SUMMARY OF 1HE INVENTION
`
`The present invention solves the problem of degradation
`of signal quality when increasing capacity on an interference
`driven channel selection/quality driven channel selection
`(IDCS/QDCS) equipped telecommunications system. The
`present invention improves signal quality by managing the
`frequencies allocated to a base station serving a cell to select
`the best of those allocated frequencies for assignment to
`base station transceivers. Measured quality metrics for cer(cid:173)
`tain ones of the allocated frequencies that are currently
`assigned to transceivers in the base station are compared
`against measured quality metrics for certain ones of the
`allocated frequencies that are currently unassigned to trans(cid:173)
`ceivers in the base station. Responsive to the results of the
`comparison, the best unassigned frequency is swapped for
`the worst assigned frequency.
`The present invention provides a method for managing
`frequencies allocated to a cell within a cellular network to
`assign the best frequencies for use by channel equipment
`within that cell. The method measures at least one quality
`metric with respect to the allocated frequencies and com(cid:173)
`pares the quality metrics for currently unassigned frequen(cid:173)
`cies against the quality metrics for currently assigned fre(cid:173)
`quencies. The best unassigned frequency is then swapped for
`the worst assigned frequency. Additionally, a voting step is
`used when comparing to indicate whether the unassigned
`frequency or the assigned frequency has a higher signal
`quality for communication.
`Another aspect of the present invention is a method for
`assigning frequencies allocated to a base station serving a
`cell within a telecommunications network. At least one
`45 quality metric for at least two of the m frequencies allocated
`to the base station are measured. There are n of the m
`allocated frequencies currently assigned to the transceivers
`of the base station and m-n frequencies currently unas(cid:173)
`signed to the transceivers of the base station. A voting step
`occurs between the n assigned and the m-n unassigned
`frequencies based on the measured metrics to indicate
`whether the currently assigned or currently unassigned fre(cid:173)
`quency is of better quality. The best m-n unassigned fre(cid:173)
`quency is then swapped for the worst n assigned frequency
`in response to a positive vote for that particular unassigned
`frequency. Additionally, a step of comparing at least one
`metric of the n assigned frequencies to the m-n unassigned
`frequencies is performed.
`Another aspect of the present invention is a system for
`arranging a set of frequencies allocated to a base station
`serving a cell within a telecommunications system. The
`system has a measuring device to measure at least one
`quality metric for a frequency currently assigned to a
`transceiver of a base station and at least one quality metric
`65 for a frequency not currently assigned to a transceiver. A
`processor operates to compare the measured quality metrics
`and exchange a certain one of the currently unassigned
`
`0008
`
`

`

`US 6,418,317 B1
`
`3
`frequencies for a certain one of the currently assigned
`frequencies based upon the relative difference of the quality
`metrics. Additionally, the processor operates to vote, which
`adds a numeric value to a memory location based upon the
`results of the comparison of the quality metrics. A filter
`within the processor may also be included to prevent the
`swapping operation from occurring before a particular event
`occurs.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`A more complete understanding of the method and appa(cid:173)
`ratus of the present invention may be acquired by reference
`to the following Detailed Description when taken in con(cid:173)
`junction with the accompanying Drawings wherein:
`FIG. 1 is an exemplary cell diagram schematically illus(cid:173)
`trating a frequency reuse cellular telephone system wherein
`the present invention is implemented;
`FIG. 2 is a simplified block diagram of a base station in
`accordance with the present invention for use in the fre(cid:173)
`quency reuse cellular telephone system of FIG. 1;
`FIG. 3 is an exemplary flow diagram to manage the
`frequencies allocated to a cell;
`FIG. 4 is an exemplary table showing each frequency
`allocated to a cell and the interference measurements for
`each frequency;
`FIG. SA is an exemplary table (VFE matrix) showing the
`results of a voting after a single sample;
`FIG. 5B is an exemplary table (VFE matrix) showing the
`results after voting for ten samples;
`FIG. 5C is an exemplary table (VFE matrix) showing the
`results after swapping an assigned frequency with an unas(cid:173)
`signed frequency;
`FIG. 5D is an exemplary table (VFE matrix) showing the
`results after swapping an assigned frequency with an unas(cid:173)
`signed frequency including results from a Mono-VFE
`matrix;
`FIG. 6A is an exemplary table (VFE matrix)showing the
`results after voting for forty samples; and
`FIG. 6B is an exemplary table (VFE matrix) showing the
`results after swapping two assigned frequencies with two
`unassigned frequencies.
`
`DETAILED DESCRIPTION OF THE DRAWINGS
`
`There are a plurality of radio frequencies in the cellular
`band available to cellular telephone system providers for use
`in communicating with mobile stations. These radio fre(cid:173)
`quencies support both traffic channels and control channels.
`The traffic channels are used for carrying telephone voice or
`data conversations. The control channels are used for car(cid:173)
`rying system operation control signals (commands). Such
`control signals include page signals, page response signals,
`location registration signals, traffic channel assignments,
`maintenance instructions, and cell selection or re-selection
`instructions.
`A cellular service area can cover a large geographic area,
`and in many instances there will be a need for a large number
`of cells that often exceeds in number the number of cells
`provided by dividing the available radio frequencies in such
`a manner as to handle expected subscriber usage.
`Accordingly, in order to provide sufficient call handling
`capacity throughout the service area, the cells are grouped
`into clusters of cells and the radio frequencies in the cellular
`band are reused in each of the clusters.
`Reference is now made to FIG. 1 for an illustration of an
`example of the frequency reuse concept commonly utilized
`
`5
`
`10
`
`4
`in cellular systems. An arbitrary geographic area (hereinafter
`"the service area") is divided into a plurality of contiguous
`cells 10 schematically represented by hexagons. The cells 10
`are then grouped into clusters 12 (outlined in bold to ease
`recognition), which in the present example comprise seven
`cells 10(1)-10(7) in each cluster. Assume for this example
`that there are a total of twenty-eight frequencies fn (wherein
`n=l to 28) available in the cellular band for simplicity, no
`virtual frequencies are shown in FIG. 1. It will of course be
`understood that each frequency actually comprises a paired
`uplink and downlink frequency. The frequencies fl-f28 are
`divided equally amongst the cells 10(1)-10(7) to provide
`four frequencies per cell. Thus, each of the cells 10(1) in the
`service area is allocated frequencies fl, f8, fl5 and f22 for
`15 carrying its traffic and control channels. Similar allocations
`are made for each of the remaining cells 10, with the
`frequencies also being reused across each of the included
`clusters 12. The complete allocation of the frequencies
`fl-f28 to the cells 10(1)-10(7) in each cluster 12 in accor-
`20 dance with this example of cellular frequency reuse is
`illustrated in detail in FIG. 1 with respect to cluster 12(1).
`It is noted in this frequency reuse scheme that in no
`instance have adjacent cells been allocated use of the same
`frequency. Reuse of an identical frequency in the service
`25 area at a minimum requires a separation of at least one cell
`10 along with a regulation of broadcast power from each cell
`to constrain radio propagation substantially within the cell
`area. Furthermore, it is noted that in no instance does any
`one cell10 utilize adjacent frequencies in the cellular band.
`30 Adjacent frequencies should exist no closer than one cell10
`away from each other. By arranging the cells 10 in clusters
`12 as shown in the figure, regulating broadcast power of
`communications within the cell, and further by allocating
`frequencies in the fashion described above and shown in the
`35 figure, the likelihood of interference is minimized while
`simultaneously providing effective and efficient cellular
`communications services across a very large service area.
`In spite of the precautions taken to avoid interference, it
`is known that interference does occur in cellular systems like
`40 that previously described. One aspect of this interference
`originates from communications occurring in the cells of
`other clusters 12 on the same frequency (i.e., co-channel
`interference). To understand this phenomena, assume the
`existence of concurrent voice communications using fre-
`45 quency flO (and perhaps individual timeslots therein) in
`each of the cells 10(3) in each of the clusters 12(2), 12(3)
`and 12(4) as shown in FIG. 1. In spite of any imposed
`broadcast power limitations, a certain amount of the radio
`frequency energy of those voice communications propagates
`50 beyond the respective cell boundaries and is injected as
`interference into frequency flO in cell10(3) of cluster 12(1).
`Another aspect of this injected interference originates
`from communications occurring in other cells on adjacent
`frequencies (i.e., adjacent channel interference) To under-
`55 stand this phenomena, assume the existence of concurrent
`voice communications on frequency f8 in cell 10(1) and
`frequency f9 in cell10(2), and perhaps individual timeslots
`therein, of cluster 12(1) as shown in FIG. 1. In spite of any
`regulations on broadcast power and the presence of guard
`60 bands around each of the frequencies, improper transceiver
`broadcasts around one frequency (for example, f8) may be
`injected as interference into the adjacent frequency f9. It
`should be recognized that adjacent channel interference is
`not nearly as common an occurrence as co-channel inter-
`65 ference in well regulated and stable communication systems.
`Because this injected interference may adversely affect
`cellular voice or data communications over a given
`
`0009
`
`

`

`US 6,418,317 B1
`
`6
`5
`frequency, it would be unwise for the system to act during
`source. The idle traffic channel signal strength measure(cid:173)
`certain times of high interference by assigning the given
`ments comprising the measured uplink interference (on
`either a frequency or timeslot basis) are reported by the
`frequency (or channel therein) to a transceiver. Selecting the
`signal strength measurement device 22 of the base station 14
`"best" channels and frequencies using channel assignment
`to either a processor 24 within the base station or the mobile
`techniques, such as quality driven channel selection (QDCS) 5
`switching center 18 (perhaps along with the hand-off traffic
`or interference driven channel selection (IDCS), may not be
`suitable for increased capacity systems. To improve perfor(cid:173)
`channel signal strength measurements made on the
`mance of higher capacity systems, it may require that a base
`frequencies/channels allocated to other cells) and are con(cid:173)
`station assign higher quality frequencies to transceivers
`sidered in connection with the process for selecting and
`within the base station using uplink and downlink signal 10
`assigning traffic channels.
`quality measurements to determine the "best" frequencies
`In one embodiment of the base station, the signal strength
`allocated to the base station to assign.
`measurement device 22 comprises a locating verification
`module (LVM) 30 including a receiver and frequency syn(cid:173)
`Each of the cells 10 in a cellular system such as that
`illustrated in FIG. 1 includes at least one base station (BS)
`thesizer for selectively tuning to any one of the frequencies
`14 configured to facilitate radio frequency communications 15
`available in the cellular band. As each of the frequencies
`with mobile stations 16 roaming throughout the service area.
`allocated to a cell 12 is subdivided into a plurality of
`The base stations 14 are illustrated as being located at or
`timeslots comprising the traffic channels, the locating veri(cid:173)
`fication module 30 further includes a circuit (not explicitly
`near the center of each of the cells 10. However, depending
`on geography and other known factors, the base stations 14
`shown) for synchronizing operation of the module to the
`may instead be located at or near the periphery of, or 20
`TDMAcommunications protocol being implemented by the
`otherwise away from the centers of, each of the cells 10. In
`system so that the signal strength measurements on a
`such instances, the base stations 14 may broadcast and
`selected frequency may be made during each of the plurality
`communicate with mobile stations 16 located within the
`of included timeslots therein. This would include not only
`cells 10 using directional rather than omni-directional anten(cid:173)
`the frequencies/timeslots allocated to and used by other
`nas. The base stations 14 are connected by communications
`25 cells, but also the frequencies/timeslots allocated to and used
`links (illustrated schematically by arrow 17) to at least one
`by the cell served by the base station 14 (i.e., assigned to the
`mobile switching center (MSC) 18 operating to control the
`channel equipment). The signal strength measurements
`made by the locating verification module 30 are then filtered
`operation of the system for providing cellular communica(cid:173)
`tions with the mobile stations 14.
`by an infinite impulse response filter (IIRF) 34 before being
`30 subsequently processed in accordance with the present
`Reference is now additionally made to FIG. 2 wherein
`invention. The filtering removes fast changes in the mea(cid:173)
`there is shown a simplified block diagram of a base station
`sured interference levels to provide a stable estimate for
`14 used in the system of FIG. 1 in accordance with the
`output and subsequent processing. With respect to the imple(cid:173)
`present invention. The base station 14 includes a plurality of
`mentation of the present invention, the idle channel super-
`transceivers (Tx/Rx) 20(1) through 20(n), wherein n is the
`35 vision functionality 32 makes and reports measurements
`number of frequencies assigned to the cell10 served by the
`made on the frequencies/timeslots associated with idle traffic
`base station. In the exemplary system shown in FIG. 1, n=4
`channels in order to supply uplink interference measurement
`wherein seven cells 10 are included in each cluster 12 and
`data.
`a total of twenty-eight available radio frequencies are
`included in the cellular band. It should be understood that
`In another embodiment of the base station, the signal
`40 strength measurement device 22 comprises an idle channel
`the present invention includes m frequencies allocated to
`supervision (ICS) functionality 32 associated with each
`each base station, so that there are m-n frequencies unas(cid:173)
`transceiver 20. The idle channel supervision functionality 32
`signed to each base station. The total set of assigned and
`unassigned frequencies is considered a virtual frequency set.
`advantageously uses the receiver portion of the transceiver
`The transceivers 20 have a configuration known in the art
`20 to make the uplink signal strength measurements. This
`45 idle channel supervision functionality 32 may include a
`that includes a transmitter and a receiver tuned to operate on
`one of the frequencies assigned to the base station 14 for its
`circuit (not explicitly shown) for synchronizing measure(cid:173)
`traffic and/or control channels. Each assigned frequency
`ment operation to the TDMA communications protocol
`provides a plurality of digital TDMA channels (i.e., plural
`being implemented by the system so that the signal strength
`full rate (FR) channels) for mobile station use.
`measurements on a selected frequency may be made during
`50 each of the plurality of included timeslots therein. The idle
`The base station 14 also includes a signal strength mea(cid:173)
`channel supervision functionality 32 makes and reports
`surement device 22 that is used in one mode known in the
`measurements made on the frequencies/timeslots associated
`art during hand-off to measure the uplink signal strength of
`with idle traffic channels in order to supply uplink interfer(cid:173)
`another station's communications on the frequency channels
`ence measurement data. In addition to interference measure-
`assigned to other cells. The operation of the signal strength
`measurement device 22 of each base station 14 is further 55 ment data, other metrics, such as power levels and timeslot
`usage can be utilized to assess frequency channel availabil(cid:173)
`controlled in an additional operating mode in accordance
`ity.
`with the present invention by received mobile switching
`center commands and/or the base station programming to
`The base stations report signal strength measurements on
`a per idle timeslot basis to either the processor 24 or the
`measure a quality metric (such as the received uplink signal
`mobile switching center 18 for processing. A measurement
`strength) for each of the frequencies supporting idle traffic 60
`processing means comprises either the processor 24 or the
`channels allocated to its own base station. These measure-
`mobile switching center 18 operating to perform the man(cid:173)
`ments are made at selected times or are made periodically in
`agement functionality of the present invention, which is to
`accordance with system specifications. The results of the
`compare the quality metrics of the measurements and swap
`idle channel measurements provide an indication of the
`65 high signal quality unassigned frequencies with low signal
`amount of injected uplink interference caused by same or
`adjacent channel communications occurring simultaneously
`quality assigned frequencies to the transceivers in the base
`within the system, or caused by any other interference
`station. By managing the allocated frequencies to the base
`
`0010
`
`

`

`US 6,418,317 B1
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`25
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`30
`
`10
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`7
`station, the base station provides the frequencies with the
`highest signal quality for subscriber communication.
`Reference is now made to FIG. 3, where an exemplary
`flow diagram representing an embodiment of the present
`invention to manage frequencies allocated to a cell10 within
`a telecommunications system is shown. The process starts at
`step 300 and at step 305, the system makes measurements of
`each frequency allocated to the cell 10. To reiterate, fre(cid:173)
`quencies are allocated to a base station 14 serving a cell10.
`For example, frequencies allocated to cell10(1) are (fl, f8,
`f15, and f22).
`A base station may have n transceivers 20, for instance,
`but the number of frequencies allocated to that base station
`14 may be m (wherein m=n+15). The n number of
`"assigned" frequencies to a cell10 is equal to the number of
`transceivers 20 within the base station 14 serving the cell10
`(i.e., n assigned frequencies). The number of unassigned
`frequencies to the cell10 is m-n (or 15 in this case). These
`unassigned frequencies are allocated and remain allocated to
`the cell 10, but are not available for use without being
`assigned in accordance with the present invention to a
`transceiver 20.
`In step 305, each allocated frequency channel (assigned
`and unassigned) within the cell 10 is measured to obtain a
`quality metric, unless an assigned frequency channel is in
`use. The type of measurements made on each frequency may
`be uplink and downlink frequency channel measurements.
`When making the frequency channel measurements, a vari(cid:173)
`ety of quality metrics on each frequency may be measured.
`These quality metrics may include, for example, signal
`strength measurements and/or interference strength mea(cid:173)
`surements.
`In making measurements on the downlink frequency
`channels, advantageous use of a mobile assisted channel
`allocation feature is provided by cellular standard IS-136 is
`made. Mobile stations 16 may be used to measure the signal
`strength on each frequency allocated to the base station 14
`serving the cell10 prior to a page response, a call origination
`or a call registration. These measurements may then be 40
`reported back to the base station 14, which may in turn
`report the measured downlink frequency channels to the
`MSC 18. Alternatively, the mobile stations 16 may be
`commanded by the base station 14 to make continuous
`measurements, for instance, once per second on the down- 45
`link frequency channels allocated to the cell 10 and report
`the measurements to the base station 14.
`In making the uplink frequency channel measurements,
`the LVM may be used to scan the list of frequencies
`allocated to the cell. The LVM may be commanded to make
`these measurements when not busy performing location and
`verification measurements. The LVM may make as many as
`50 frequency (150 timeslot) measurements per second or
`more.
`When making measurements on the frequencies that are
`assigned to the base station 14, in general, the measurements
`are made on the current idle assigned frequencies. It should
`be understood that a system having multiple timeslots per
`frequency channel (TDMA) may make measurements on the
`individual idle timeslots. The measurements may also be
`made over a moving time window.
`At step 310, the signal quality measurements are com(cid:173)
`pared. The measurements that are compared are the quality
`metric measurements made on unassigned frequency chan(cid:173)
`nels against the quality metric measurements made on
`assigned idle frequency channels. It should be noted that the
`unassigned downlink frequency channels are compared to
`
`8
`the assigned idle downlink frequency channels and are not
`compared to the assigned idle uplink frequency channels. In
`making a comparison, it is typical to add a certain number
`of decibels (dB) to unassigned frequency measurements to
`5 account for a hysteresis. The results of the comparison
`indicate whether the quality metrics of any unassigned
`frequency channel is better (e.g., include less injected
`interference) than the quality metrics of any assigned idle
`frequency channel.
`At step 315, a vote is made for the unassigned frequency
`channel or the assigned idle frequency channels based upon
`the results of the measurements compared at step 310. The
`step of voting 315 basically adds and subtracts numerical
`values in a virtual frequency exchange (VFE) matrix or
`memory location after each measurement sample. Each of
`15 these numeric values may be a fixed value (e.g., the value 1),
`the actual difference value in decibels, a difference of the
`average value over a number of sample periods or time
`interval, or a percentage differen