throbber
United States Patent
`
`[19]
`
`Frodigh et al.
`
`[54]
`
`[75]
`
`[73]
`
`[21]
`
`[22]
`
`[51]
`[52]
`[58]
`
`ADAPTIVE CHANNEL ALLOCATIONIN A
`FREQUENCYDIVISION MULTIPLEXED
`SYSTEM
`
`Inventors: Carl Magnus Frodigh; Perols Leif
`Mikael Gudmundson. both of Kista,
`Sweden
`
`Assignee: Telefonaktiebolaget L M Ericsson
`publ., Stockholm. Sweden
`
`Appl. No.: 493,489
`
`Filed:
`
`Jun. 22, 1995
`
`Tint. CMS oncecessesserseerceneensceecennecessnncesenen H04J 1/16
`
`
`US. CL...
`. 370/252; 370/333; 455/63
`
`Field of Search ou...ccssecenecsees 370/17, 13, 95.1.
`370/95.2, 95.3, 71. 57, 19, 18, 30, 73, 69.1,
`247, 252, 251. 254, 314, 332, 333, 337,
`341, 344, 345, 346, 350, 281, 320, 335,
`342, 478, 479, 480, 481; 375/200-205,
`227, 254, 278, 284, 285, 293; 455/33.1-33.3.
`34.1, 54.1, 50.1, 51.1, 52.2, 52,3, 57.1,
`63. 65. 67, 67.1. 67.3. 67.4, 101; 379/58-60
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`US005726978A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,726,978
`Mar. 10, 1998
`
`3/1994 Greenberg et al. cssssssssessssnese 370/57
`5,295,138
`5/1994 Fouche et al.
`ssesssssosssssseesenne 375/260
`5,311,550
`
`6/1994 Hamabe et al. o...ssesssssensssen 455/33.2
`5,319,795
`...ssscsssesserssesnse 370/19
`9/1994 Gledhill et ab.
`5,345,440
`
`3/1996 Lagerqvist et ab...
`eee 370/17
`5,502,713
`
`cseesscaseecssecsscunssessssrseneseee 370/17
`5513174 1996 Pumj
`OTHER PUBLICATIONS
`
`Leonard J. Cimini. Jr.. IEEE Transactions on Communica-
`tions, vol. COM-33, No. 7, Jul. 1985, “Analysis and Simu-
`lation of a Digital Mobile Channel Using Orthogonal Fre-
`quency Division Multiplexing”, pp. 665-675.
`M. Alard and R. Lassalie, EBU Review, technical No. 224.
`Aug. 1987, “Principles of Modulation and Channel Coding
`for Digital Broadcasting for Mobile Receivers”. pp. 47-69.
`D. DiZenobio, G. Santella and Fondazione Ugo Bordoni,
`The Fourth International Symposium on Personal, Indoor
`and Mobile Radio Communications, Yokahama. Japan. Sep.
`8-11. 1993, “OFDM Technique for Digital Television
`Broadcasting to Portable Receivers”, pp. 244-248.
`Hakan Eriksson and Randall Bownds, IEEE 1991. “Perfor-
`mance of Dynamic Channel Allocation in the DECT Sys-
`tem”, pp. 693-698.
`
`(List continued on next page.)
`
`Primary Examiner—Dang Ton
`Attorney, Agent, or Firm—Jenkens & Gilchrist, PC; Brian T.
`Rivers
`
`[57]
`
`ABSTRACT
`
`......scccssssessssscere 370/71
`2/1975 DeLong et all.
`3,864,521
`.. 370/30
`7/1985 Morais ........
`4,528,656
`A method and system of adaptive channel allocation in a
`4/1988 Schloemer .........uscsseneesseseeesnee 455/33
`4,736,453
`
`frequency division multiplexed system is provided. In the
`14/1988 Takahata et al.
`...
`«» 370/69.1
`4,783,779
`5/1990 Baxter et ab.
`.....cccsscorssreseeasee 370/73
`4,930,120
`method and system a subset of M subcarriers is chosen from
`
`a larger set of N subcarriers available for communications on
`5,014,342—S/1991 Pudsey ........sccscssonsssseconessnstscssee 455/33
`5,109,529
`4/1992 Akaiwa.......
`ww 455/34.1
`a link. As communications take place on the link. signal
`
`w 370/95.1
`5,161,154
`11/1992 Diaz et al.
`......
`quality (C/I) measurements on the subcarriers of the subset
`
`........csscsesseeesses 370/18
`5,191,576
`3/1993 Pommmieret al.
`of M subcarriers and interference (I) measurements on the
`5,203,012
`4/1993 Patsiokas et all.
`.....csescaecee 455/34.1
`subcarriers of the group of N subcarriers are periodically
`8/1993 Strawcynski etal.
`« 455/54.1
`5,239,682
`
`performed. The C/I and I measurements are then used to
`9/1993 Mulford .........00
`« 455/34.1
`5,249,304
`
`reconfigure the subset of M subcarriers to reduce co-channel
`wore 375/200
`5,260,968
`11/1993 Gardneret al.
`.
`interference on the link.
`5,263,176
`11/1993 Kojima etal. ......
`« 455/34.1
`5,282,222
`1/1994 Fattouche et al.
`..
`were 375/200
`
`5,289,464—2/1994 Wang ...rcccrsrssssesreseseareeceerseane 370/69.1
`22 Claims, 8 Drawing Sheets
`
`RECEIVE MEASUREMENT RESULTS
`f-
`FORM AVAILARLE SUBCARRIERS
`
`
`DETERWINC M LESS!
`INTERFERED
`§°
`UNUSED SUBCARRIERS
`
`SIND SUBSET ASSICNMITN?
`TO URE RICHER
`
`SAMSUNG 1016
`
`
`
`TACHANGE LEAST INTERFERED
`UNUSEL SURCARRIER WII SUASET
`SUBCARGICR wile LOWEST C,
`
`
` 1
`
`ff
`
`__ACCEIE MCASUREME#1
`RESULTS. FROM LOR RECEIVER
`
`OR RECEIVE CELL ENO
`
`
`
`
`
`
`1
`
`SAMSUNG 1016
`
`

`

`5,726,978
`Page 2
`
`OTHER PUBLICATIONS
`
`Reiner Beck and Herbert Panzer, IEEE 1989, “Strategies for
`Handover and Dynamic Channel Allocation in Micro—Cel-
`lular Mobile Radio Systems”. pp. 178-185.
`Syuji Yasuda and Seizo Onoe, IEEE 1992. “Autonomous
`Channel Assignment Control for Flexible Reuse in Mobile
`Radio Systems”, pp. 798-801.
`
`Hakan Andersson, Hakan Eriksson, Anna Fallgren and Mag-
`nus Madfors, IEEE 1992. “Adaptive Channel Allocation in
`a TIA IS-54 System”, pp. 778-781.
`Hakan Eriksson, IEEE 1988, “Capacity Improvement by
`Adaptive Channel Allocation”, pp. 1355-1359.
`Eduardo F. Casa and Cyril Leung, IEEE 1991, “OFDM for
`Data Communication Over Mobile Radio FM Channels—
`Part I: Analysis and Experimental Results”. pp. 783-793.
`
`2
`
`

`

`US. Patent
`
`Sheet 1 of 8
`
`Mar. 10, 1998
`
`5,726,978
`
`3
`
`

`

`USS. Patent
`
`Mar. 10, 1998
`
`Sheet 2 of 8
`
`5,726,978
`
`202
`
`MOBILE
`STATION
`
`BASE
`STATION
`
`STATION
`
`MOBILE
`
`304
`
`306
`
`508
`
`SYSTEM
`
`4
`
`

`

`US. Patent
`
`Mar. 10, 1998
`
`Sheet 3 of 8
`
`5,726,978
`
`
`
`342
`ACA PROCESSING
`
`
`
`PROCESSING MEANS
`TO
`ACA PROCESSING
`
`
`
`FROM STEP 426 OF FIG. 4A
`
`4280
`
`SHORT
`TIME INTERVAL FOR
`
`
`
`REPORTING
`SEND C/I
`AND |
`
`MEASURMENTS
`
`
` SEND Y LOWEST C/I
`TO SYSTEM
`
`
`MEASURMENTS AND
`
`Z LOWEST |
`
`MEASURMENTS TO SYSTEM
`
`
`TO STEP 414 OF FIG. 4A
`
`FIG. 4B
`
`426b
`
`5
`
`

`

`U.S. Patent
`
`Mar.10, 1998
`
`Sheet 4 of 8
`
`5,726,978
`
`AVAILABLE SUBCARRIERS
`
`402
`
`404
`
`406
`
`FIG.
`
`4A
`
`(var)
`
`408
`
`RECEIVE ASSIGNMENT OF
`SUBSET OF M SUBCARRIERS
`
`RECEIVE ON LINK
`USING ASSIGNED SUBSET
`
`410
`
`12
`
`4
`
` RECEIVE CALL END, RECONFIGURE,
`OR MEASUREMENT TIMER MESSAGE
`
`
`
`RECONFIGURE
`SUBSET OF M
`
`SUBCARRIERS AS
`PER MESSAGE
`
`6
`
`

`

`U.S. Patent
`
`Mar. 10, 1998
`
`Sheet 5 of 8
`
`5,726,978
`
`FIG. 5
`
`RECEIVE MEASUREMENT RESULTS/-502
`FOR N AVAILABLE SUBCARRIERS
`
`DETERMINE M LEAST INTERFERED[~204
`UNUSED SUBCARRIERS
`
`SEND SUBSET ASSIGNMENT
`TO LINK RECEIVER
`
`506
`
`RECEIVE MEASUREMENT
`RESULTS FROM LINK RECEIVER
`OR RECEIVE CELL END
`
`
`
`
`
`
`
`DETERMINE SUBCARRIER
`OF SUBSET WITH LOWEST C/I
`
`
`
`
`EXCHANGE LEAST INTERFERED
`UNUSED SUBCARRIER WITH SUBSET
`SUBCARRIER WITH LOWEST C/I
`
`SEND RECONFIGURE MESSAGE
`TO LINK RECEIVER
`
`
`
`7
`
`

`

`U.S. Patent
`
`Mar.10, 1998
`
`Sheet 6 of 8
`
`5,726,978
`
`RECEIVE MEASUREMENT ORDER/2~60
`2
`
`FIG.
`
`6A
`
`MEASURE | ON N
`AVAILABLE. SUBCARRIERS
`
`DETERMINE M LEAST
`INTERFERED SUBCARRIERS
`
`604
`
`606
`
`SEND SUBSET REQUEST TO SYSTEM
`
`608
`
`RECEIVE ANSWER FROM SYSTEM
`
`612
`
`
`
`
`
`DETERMINE NEXT
`
`CANDIDATE(S) FOR SUBSET
`
`
`TRANSMIT SUGGESTED
`
`CANDIDATE(S) 10 SYSTEM
`
`
`
`618
`
`YES
`RECEIVE ON LINK
`USING SUBSET
`
`620
`
`LA)
`
`FROM A OF FIG. 6B
`
`622
`
`RECEIVE CALL END OR
`MEASUREMENT TIMER MESSAGE
`
`6
`
`24
`
`YES Gut>626
`
`END
`
`NO
`
`MEASURE | ON ALL N
`SUBCARRIERS AND DO AVERAGING
`
`MEASURE C/I ON SUBSET OF M
`SUBCARRIERS AND DO AVERAGING
`
`628
`
`630
`
`TO B OF FIG. 6B
`
`8
`
`

`

`U.S. Patent
`
`Mar.10, 1998
`
`Sheet 7 of 8
`
`5,726,978
`
`FROM B OF 6A
`
`632
`
`634
`
`FIG. 6B
`
`
`
`DETERMINE SUBCARRIER OF
`SUBSET WITH LOWEST C/I
`
`
`C/I
`
` NO
`BELOW
`
`
`THRESHOLD?
`
`
`TO A OF
`FIG. 6A
`
`
`
`LESS
`INTERFERED
`
`
`SUBCARRIER?
`
`TRANSMIT SUBCARRIER
`REQUEST TO SYSTEM
`
`
`642
`
`
`646
`
`
`
`FIND LESS INTERFERED SUBCARRIER
`OTHER THAN REJECTED REPLACEMENT
`
`
`
`
`INTERFERED
`SUBCARRIER?
`
`RECONFIGURE
`SUBSET
`
`9
`
`

`

`U.S. Patent
`
`Mar. 10, 1998
`
`Sheet8 of 8
`
`5,726,978
`
`FIG. 7
`
`RECEIVE SUBSET REQUEST MESSAGE
`
`702
`
`
`
`
`
`ALL
`REQUESTED
`SUBCARRIERS
`AVAILABLE?
`YES
`E
`
`TRANSMIT SUBCARRIFR
`ACCEPTED MESSAGE(S)
`
`
`
`706
`
`710
`
`TRANSMIT SUBSET OR
`SUBCARRIER ACCEPTED MESSAGE
`
`RECEIVE SUBCARRIER REQUEST
`OR CALL END MESSAGE
`
`
`
` REQUESTED
`SUBCARRIER
`AVAILABLE?
`
`NO
`
`10
`
`

`

`5.726.978
`
`1
`ADAPTIVE CHANNEL ALLOCATIONIN A
`FREQUENCYDIVISION MULTIPLEXED
`SYSTEM
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`This invention relates to cellular telecommunications sys-
`tems and, more particularly. to a method and system of
`adaptive channel allocation in a frequency division multi-
`plexed cellular system.
`2. Description of the Prior Art
`In a cellular telecommunications system the user of a
`mobile station communicates with the system through a
`radio interface while moving about the geographic coverage
`area of the system. The radio interface between the mobile
`station and system is implemented by providing base sta-
`tions dispersed throughout the coverage area of the system,
`each capable of radio communication with the mobile sta-
`tions operating within the system. In a typical cellular
`telecommunications system each base station of the system
`controls communications within a certain geographic cov-
`erage area termed a cell, and a mobile station which is
`located within a particular cell communicates with the base
`station controlling that cell. As a mobile station moves
`throughout
`the system control of the communication
`between the system and mobile station are transferred from
`cell to cell according to the movementof the mobile station
`throughout the system.
`Existing cellular telecommunications systems operate
`according to various air interface standards which assure the
`compatibility of equipment designed to operate in a particu-
`lar system. Each standard provides specific details of the
`processes that take place between the mobile stations and
`base stations of the system in all modes of operation.
`including during idle states, during rescan of control
`channels, during registration, and during connection to voice
`or traffic channels. Advancesin cellular systems technology
`have been rapid in recent years. These advancesin technol-
`ogy have been driven by increases in demand for the
`increasingly sophisticated services offered by cellular sys-
`tems. As cellular systems technology and the total number of
`cellular systems has increased worldwide to meet
`this
`demand, there has also been an accompanying increase in
`the number of system standards according to which these
`cellular systems operate.
`In cellular telecommunications systems, as in most radio
`systems,
`the frequency bandwidth available for use is a
`limited resource. Because of this, emphasis is often concen-
`trated on making the most efficient use possible of the
`available frequency bandwidth when developing new cellu-
`lar systems. Additionally, communications within cellular
`systems are often subject to certain types of RF signal
`distortion such as multipath propagation and co-channel
`interference. The development of new system standards has
`also emphasized the need to minimize the effect of these RF
`signal distortions on communications within the cells of a
`system.
`Frequency division multiplexing (FDM) is a method of
`transmitting data that has application to cellular systems.
`Orthogonal frequency division multiplexing (OFDM)is a
`particular method of FDM that is particularly suited for
`cellular systems. An OFDM signal consists of a number of
`subcarriers multiplexed together, each subcarrier at a differ-
`ent frequency and each modulated by a signal which varies
`discretely rather than continuously. Because the level of the
`modulating signal varies discretely, the power spectrum of
`
`2
`each subcarrier follows a (sin x/x)* distribution. The spectral
`shape transmitted on each subcarrier is such that the spectra
`of the individual sub-channels are zero at the other subcar-
`tier frequencies and interference does not occur between
`subcarriers. Generally. N serial data elements modulate N
`subcarrier frequencies, which are then frequency division
`multiplexed. Each of the N serial data elements comprises a
`data block with a duration of T=1/fs, where fs is the
`bandwidth of the OFDM signal. The subcarriers of the
`OFDMsystem are separated in frequency by multiples of
`1/T. Although the frequency spectrum of the subcarriers
`overlap,
`this frequency spacing makes the subcarriers
`orthogonal over one symbol interval so that the peak of
`power of each modulated carrier occurs at frequencies
`corresponding to nulls in the power spectrum of the other
`carriers. The overall spectrum of an OFDM signal is close to
`rectangular when a large number of OFDM carriers are
`contained in the OFDM signal.
`During the time period, T. the OFDM signal may be
`represented by a block of N samples. The value of the N
`samples is as follows:
`
`yas xa etinkn
`wn = 2 (k)
`
`The N values X(k) represent the respective data during
`period T, of the discretely-varying signals modulating the
`OFDMcarriers e?”"*”", From the above, the OFDM signal
`correspondsto the inverse Discrete Fourier Transform of the
`set of data samples X(k). To convert a data stream into an
`OFDMsignal, the data stream is split up into blocks of N
`samples X(k) and an inverse Discrete Fourier Transform is
`performed on each block. The string of blocks that appears
`at a particular sample position over time constitutes a
`discretely-varying signal that modulates a certain subcarrier
`at a frequency fn.
`OFDMoffers several advantages that are desirable in a
`cellular system. In OFDM the orthogonality of the subcar-
`riers in the frequency spectrum allows the overall spectrum
`of an OFDM signal to be close to rectangular. This results in
`efficient use of the bandwidth available to a system. OFDM
`also offers advantages in that interference caused by multi-
`path propagation effects is reduced. Multipath propagation
`effects are caused by radio wavescattering from buildings
`and other structures in the path of the radio wave. Multipath
`propagation may result in frequency selective multipath
`fading. In an OFDM system the spectrum of each individual
`data element normally occupies only a small part of the
`available bandwidth. This has the effect of spreading out a
`multipath fade over many symbols. This effectively random-
`izes burst errors caused by the frequency selective multipath
`fading, so that instead of ome or several symbols being
`completely destroyed, many symbols are only slightly dis-
`torted. Additionally, OFDM offers the advantage that the
`time period T may be chosen to be relatively large as
`compared with symbol delay time on the transmission
`channel. This has the effect of reducing intersymbol inter-
`ference caused by receiving portions of different symbols at
`the same time.
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`The use of OFDMin cellular systems has been proposed
`by Cimini, “Analysis and Simulation of a Digital Mobile
`Channel Using Orthogonal Frequency Division
`Multiplexing”, JEEE Trans. Commun.. Vol. 33, No. 7. pp.
`665-675 (July, 1985). A similar application of OFDM in a
`mobile system has also been proposed by Casa. “OFDM for
`Data Communication Over Mobile Radio FM-Channels-
`
`65
`
`11
`
`11
`
`

`

`5.726.978
`
`3
`Part I: Analysis and Experimental Results”, IEEE Trans.
`Commun.. Vol. 39, No. 5, pp. 783-793 (May, 1991). In these
`OFDM cellular systems a set of subcarrier frequencies is
`assigned to each communications link created for transmis-
`sion from a base station to a mobile station (downlink) and
`from a mobile station to a base station (uplink) operating
`within a cell. The set of subcarrier frequencies allocated to
`each communications link is chosen from all subcarrier
`frequencies available to the system. Within a cell the same
`subcarrier frequency cannot be assigned to more than one
`communications link. Thus, co-channel
`interference
`between subcarriers within the same cell does not occur.
`However, it is possible in such an OFDM system that a
`communications link in a cell of the system is assigned a set
`of subcarriers frequencies that includes one or more subcar-
`riers frequencies also assigned to a communicationslink set
`up in another cell within the system. Each of these com-
`monly assigned subcarriers frequencies may be subject to
`co-channel
`interference caused by the use of the same
`subcarrier frequency in the other cells. In these OFDM
`systems no method or system exists for coordinating the
`assignment of subcarrier frequencies to communications
`links created within different cells. In such a system the
`co-channelinterference in a communications link caused by
`a subcarrier used in a neighboring cell could be very large.
`Methods ofallocating channel frequencies among cells in
`non-OFDM systems have been developed that reduce or
`minimize co-channel interference. Adaptive Channel Allo-
`cation (ACA) is such a method. In ACA any channel
`frequency allocated to a cellular system may beusedto set
`up a link in any cell of the system regardless of whether or
`not the frequency is used elsewhere in the system aslong as
`certain interference criteria are met. The channel frequencies
`may also be freely reused throughout the system as long as
`the interference criteria are met.
`In Adaptive Channel Allocation various measurements of
`signal quality and interference levels on dynamically allo-
`cated channel frequencies are performed within the coverage
`area of a cell to build a list of traffic or voice channels that
`may be assigned to communications links to be created
`within the cell. The base station controlling the cell and
`mobile stations within the cell’s coverage area perform
`measurements on the set of channel frequencies that the
`system operator has allocated to be dynamically allocated
`for communications within the system. Generally. both
`uplink and downlink measurements are performed. Based on
`these measurements, when a new link is to be created. a
`channelfrequency is assigned to the link based on somerule.
`For example, in minimum interference ACA the system
`builds a table of channels from the least interfered (highest
`quality) to the most interfered (lowest quality) channels as
`measured within each cell. The system then selects a certain
`number ofleast interfered channel frequencies from thatlist
`to allocate to communicationin that cell. Other criteria, such
`as certain required frequency separation between the chan-
`nels chosen and avoiding certain combinations of channels
`whose frequencies create intermodulation are also consid-
`ered. As an example of ACA. H. Eriksson, “Capacity
`improvement by Adaptive Channel Allocation”, IEEE Glo-
`bal Telecomm. Conf.. pp. 1355-1359, Nov. 28—Dec.1. 1988,
`illustrates the capacity gains associated with a cellular radio
`system where all of the channels are a common resource
`shared by all base stations. In the above-referenced report.
`the mobile measuresthe signal quality of the downlink, and
`channels are assigned on the basis of selecting the channel
`with the highest carrier to interference ratio (C/I level).
`Existing ACA algorithms which have been created for
`non-OEDMcellular systems using one carrier frequency for
`
`4
`each link cannot be effectively used in a cellular system
`using OFDM. One problem with the existing ACA tech-
`niquesis that the numberof subcarriers in an OFDM system
`is large compared to the numberofcarriers in the system that
`uses a single carrier for each communications link. This
`requires an extensive measurementeffort that expends both
`time and system resourcesto obtain the uplink and downlink
`measurement results necessary for ACA. In addition, in
`orderto transfer the results of the large number of downlink
`measurements made at a mobile station to the system for
`processing, use of a large amount of signaling resources is
`necessary.
`It would provide an advantage then, to have a method and
`system of adaptive channelallocation for use in an OFDM
`system. The method and system should provide an allocation
`of subcarriers within an OFDM system that
`lessens
`co-channel interference between cells of the system. The
`method and system should also be designed to take into
`account the unique features of the OFDM system in order to
`utilize system resources effectively when allocating chan-
`nels. The present invention provides such a method and
`system.
`
`SUMMARYOF THE INVENTION
`
`The present invention provides a method and system of
`adaptive channel allocation (ACA) in an orthogonal fre-
`quency division multiplexed (OFDM)system. The method
`and system provides an allocation of subcarriers to each link
`of the OFDM system that lessens co-channel interference
`between cells of the system.
`The present invention also overcomesthe difficulties and
`shortcomings presented with implementing conventional
`ACA methods and systems designed for use in a non-OFDM
`system into an OFDM system. Conventional ACA methods
`are designed to adaptively allocate RF channels to systems
`where one channelis used per link. As applied to an OFDM
`system, these conventional ACA methods would require that
`all OFDM subcarriers assigned to users to be adaptively
`allocated. Adaptively allocating all OFDM subcarriers in an
`OFDM system would require an overly large amount of
`measurement and signaling resources to transfer channel
`measurement information and the assignment information
`between receivers and transmitters of the system. By selec-
`tively choosing the subcarriers to be adaptively allocated,
`andsetting criteria for allocation determination, the system
`and method of the present invention minimizes the use of
`measurement and signaling resources while still providing
`effective ACA.
`In a first aspect of the invention. an initial subset of M
`subcarriers is chosen from a larger group of N subcarriers
`that are available for communications on each separate link
`of the OFDM system. The number M depends on the data
`rate of the particular link and may vary betweenthe links of
`the system. The subset of M subcarriersis then used to carry
`communicationson the link. As communications take place.
`the signal quality level (C/I) of the subcarriers within the
`subset of M subcarriers, and the interference level (I) ofall
`N available subcarriers is periodically measured. These C/I
`and I measurementresults are reported to the system. During
`communications on the link the system determines from the
`C/I and I measurements if a more preferred unused subcar-
`rier which would give better signal reception on the link than
`a subcarrier of the set of M is available in the cell within
`whichthelink exists.If it is determined that a more preferred
`unused subcarrier exists, the system reconfigures the subset
`of M subcarriers to include the unused subcarrier.
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`In a second aspect of the invention, a mobile station as
`link receiver transmits only a limited set of measurement
`results to the system at certain select reporting intervals
`rather than all measurementresults. The transmitted limited
`set of measurementresults comprises a select number of the
`lowest C/I measurementresults and a select number of the
`lowest I measurement results. The transmission of the lim-
`ited set of results reduces the use of uplink system signaling
`Tesources.
`
`In an alternative embodiment of the invention a mobile
`station as link receiver periodically measures the signal
`quality level (C/T) of the subcarriers within the subset of M
`subcarriers, and the interference level (1) of all N available
`subcarriers. The mobile station then determines candidate
`replacement subcarriers for the link based on the C/I and I
`measurements, and transmits a subcarrier request message to
`the system requesting that
`the candidate subcarrier be
`assigned to replace a subcarrier of the link The system
`responds to the subcarrier request message with a subcarrier
`accepted or subcarrier rejected message. If a subcarrier
`accepted message is received, the mobile station reconfig-
`ures the subset of M subcarriers to contain the candidate
`replacement subcarrier. If the subcarrier is rejected, the
`mobile station transmits a subcarrier request message
`Tequesting a new candidate subcarrier.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG.1 illustrates a cellular telecommunications network
`within which the present invention may be implemented;
`FIG.2A illustrates the allocation of subcarriers in accor-
`dance with the present invention in an orthogonal frequency
`division multiplexed system;
`FIG. 3A is a block diagram of a system according to an
`embodiment of the present invention;
`FIGS. 3B and 3C are block diagrams of a link transmitter
`and link receiver. respectively, according to an embodiment
`of the present invention;
`FIGS. 4A and 4B are flow diagrams of process steps
`according to an embodimentof the present invention per-
`formed by a link receiver;
`FIG.5 is a flow diagram of process steps according to an
`embodiment of the present invention performed within a
`cellular telecommunications network;
`FIGS. 6A and 6B are flow diagrams of process steps
`according to an alternative embodiment of the present
`invention performed by a link receiver; and
`FIG.7 is a flow diagram of process steps according to an
`alternative embodiment of the present invention performed
`within a cellular telecommunications system.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`there is illustrated a frequency
`Referring to FIG. 1.
`division multiplexed (FDM) cellular telecommunications
`system of the type to which the present invention generally
`pertains. In FIG. 1, an arbitrary geographic area may be
`divided into a plurality of contiguous radio coverage areas,
`or cells C1~C10. While the system of FIG. 1 is illustratively
`shown to include only 10 cells, it should be clearly under-
`stood that in practice. the number of cells will be much
`larger.
`Associated with and located within each of the cells
`C1-C19is a base station designated as a corresponding one
`of a plurality of base stations BI-B10. Each of the base
`stations B1-B10 includes a transmitter, a receiver, and a
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`base station controller as are well knownin the art. In FIG.
`1, the base stations B1—-B16 areillustratively located at the
`center of each of the cells C1-C10, respectively, and are
`equipped with omni-directional antennas. However. in other
`configurations of the cellular radio system, the base stations
`B1-B10 may be located near the periphery. or otherwise
`away from the center of the cells C1-C10 and mayillumi-
`nate the cells CI-C10 with radio signals either omni-
`directionally or directionally. Therefore, the representation
`of the cellular radio system of FIG. 1 is for purposes of
`illustration only and is not intended as a limitation on the
`possible implementations of the cellular telecommunica-
`tions system within which the present invention is imple-
`mented.
`
`With continuing reference to FIG.1, a plurality of mobile
`stations MI-M10 may be found within the cells C1-C19.
`Again. only 10 mobile stations are shown in FIG. 1 butit
`should be understood that the actual number of mobile
`stations will be much larger in practice and will invariably
`greatly exceed the number of base stations. Moreover, while
`none of the mobile stations MI-M10 may be found in some
`of the cells C1-C10. the presence or absence of the mobile
`stations M1—M10 in any particular one of the cells CI-C10
`should be understood to dependin practice on the individual
`desires of the users of mobile stations M1-M10 who may
`roam from one location in the cell to another or from one cell
`to an adjacent cell or neighboring cell, and even from one
`cellular radio system served by a particular MSC to another
`such system.
`Each of the mobile stations MI-M10 is capable of
`initiating or receiving a telephone call through one or more
`of the base stations B1-B16 and a mobile station switching
`center MSC. A mobile station switching center MSC is
`connected by communication links. e.g., cables. to each of
`the illustrative base stations B1—B10 and tothe fixed public
`switched telephone network PSTN, not shown. or a similar
`fixed network which may include an integrated system
`digital network (ISDN) facility. The relevant connections
`between the mobile station switching center MSC and the
`base stations B1-B10, or between the mobile station switch-
`ing center MSC and the PSTN or ISDN,are not completely
`shownin FIG. 1 but are well knownto those of ordinary skill
`in the art. Similarly, it is also known to include more than
`one mobile station switching center in a cellular radio
`system and to connect each additional mobile station switch-
`ing center to a different group of base stations and to other
`mobile station switching center via cable or radio links.
`Each MSC maycontrol in a system the administration of
`communication between each of the base stations B1-B10
`and the mobile stations M1—M10 in communication with it.
`As a mobile station roams about the system. the mobile
`station registers its location with the system through the base
`station that controls the area in which the mobile station is
`located. When the mobile station telecommunications sys-
`tem receives a call addressed to a particular mobile station,
`a paging message addressed to that mobile station is broad-
`cast on control channels of the base stations which control
`the area in which the mobilestation is believed to be located.
`Upon receiving the paging message addressed to it, the
`mobile station scans system access channels and sends a
`Page responseto the base station from whichit received the
`strongest access channel signal. The process is then initiated
`to create the call connection. The MSC controls the paging
`of a mobile station believed to be in the geographic area
`served by its base stations B1-B10 in responseto the receipt
`of a call for that mobile station, the assignment of radio
`channels to a mobile station by a base station upon receipt
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`of a page response from the mobile station, as well as the
`handoff communications with a mobile station from one
`base station to another in response to the mobile station
`traveling through the system. from cell to cell, while com-
`munication is in progress.
`Eachof the cells C1-C10is allocated a plurality of FDM
`subcarrier frequencies and at least one dedicated control
`channel. The control channel is used to control or supervise
`the operation of mobile stations by means of information
`transmitted to and received from those units. Such informa-
`tion may include incoming call signals, outgoing call
`signals, page signals. page response signals. location regis-
`tration signals and voice andtraffic subcarrier assignments.
`The present
`invention involves implementation of a
`method and system of adaptive channel allocation (ACA)
`into an FDM cellular system as shown in FIG. 1. In an
`exemplary embodiment of the invention, ACA is imple-
`mented into an OFDM system operating with a total system
`bandwidth of 5 MHz and a subcarrier spacing of 5 KHz. The
`total number of subcarriers available for this system is
`approximately 5 MHz/S KHz=1000. The subcarriers are
`modulated onto a system RF carrier with a frequency of 2
`GHz for transmission over the system RF channel and the
`frequency spectra of the transmitted signal
`is centered
`around the RF carrier. All subcarriers are available for use in
`eachcell but a subcarrier may not be used simultaneously on
`more than one link in a cell. Frequency division duplex
`(FDD) is used for separation of the uplink and downlink
`subcarriers frequencies. The system includes a dedicated
`control channel (DCCH)thatis both an uplink and downlink
`channelfor transmitting control information for handovers,
`long term channelallocation information, long term power
`control information and measurement messages and mea-
`surementresults. The system also includes a physical control
`channel (PCCH)thatis both an uplink and downlink channel
`for transmitting short term channel allocation information.
`short term power control information, measurement mes-
`sages and measurement results.
`In the ACA of the invention, for each up/down link
`between a mobile station and base station,
`the system
`chooses a subset of a number (M) of subcarriers from a set
`of a number (N) of subcarriers. The set of N subcarriers is
`the set of subcarriers available within the system for each
`link, where N>M.Theset of N subcarriers does not change
`during a communication. The set of N subcarriers may
`include all subcarriers of the system. Alternatively. the set of
`N subcarriers may be a set less in number than the total
`number of subcarriers available but greater in number than
`the number of carriers in the subset of M subcarriers.
`Referring now to FIG. 2 therein is illustrated the alloca-
`tion of subcarriers in accordance with the present invention
`in an OFDM system.Basestation 200 communicates with
`mobile station 202 over downlink 206 and uplink 208. Base
`station 200 also communicates with mobile station 204 over
`downlink 210 and uplink 212. Transmissions on links 206.
`208, 210 and 212 are made over the system RF channel.
`Voice and data to be transmitted on each link are modulated
`onto a number (M) subcarriers. The M subcarriers are then
`modulated onto the system RF carrier for transmission over
`the system RF channel. Bach link 206, 208, 210 and 212.
`within the cell uses a separate subset of M subcarriers. The
`subcarriers can only be us

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