United States Patent
`US 6,947,748 B2
`(10) Patent No.:
`(12)
`Li et al.
`(45) Date of Patent:
`Sep. 20, 2005
`
`
`US006947748B2
`
`(54) OFDMA WITH ADAPTIVE SUBCARRIER-
`CLUSTERCONFIGURATIONAND
`Inventors: Xiaodong Li, Bellevue, WA (US); Hui
`Liu, Sammamish, WA (US); Kemin Li,
`Bellevue, WA (US); Wenzhong Zhang,
`Bellevue, WA (US)
`
`(75)
`
`(73) Assignee: Adaptix, Inc., Bothell, WA (US)
`
`(*) Notice:
`
`Subject to any disclaimer, the term ofthis
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 765 days.
`
`(21) Appl. No.: 09/738,086
`.
`(22)
`Filed:
`Dec.15, 2000
`(65)
`Prior Publication Data
`US 2002/0119781 A1 Aug. 29, 2002
`Int. C1?cece H04Q 7/20; HO4J 11/00
`(51)
`(52) U.S. Ch. eee 455/450; 455/447; 455/453;
`455/455; 455/464; 370/208
`(58) Field of Search oo... cece 455/447-453,
`455/456.5, 456.6, 509, 512, 513, 524-526,
`62, 63.1, 455, 463, 464, 168.1, 176.1, 179.1,
`188.1, 550.1, 553; 370/311, 347, 480, 203-206,
`208, 210, 329; 375/132
`
`(56)
`
`References Cited
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`tion Scheme for Fixed and Mobile Receives,” IEEE Trans-
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`
`(Continued)
`
`Primary Examiner—William Trost
`Assistant Examiner—Meless Zewdu
`(74) Attorney, Agent, or Firm—Fulbright & Jaworski LLP
`
`(57)
`
`ABSTRACT
`
`Amethod and apparatus for subcarrier selection for systems
`is described.
`In one embodiment,
`the system employs
`orthogonal frequency division multiple access (OFDMA). In
`one embodiment, a method for subcarrier selection com-
`prises each of multiple subscribers measuring channel and
`interference information for subcarriers based on pilot sym-
`bols received from a basestation, at least one of subscribers
`:
`.
`.
`a:
`selecting, a set of candidate subcarriers, providing feedback
`information on the set of candidate subcarriers to the base
`station, and the one subscriber receiving an indication of
`subcarriers of the set of subcarriers selected by the base
`station for use by the one subscriber.
`
`(Continued)
`
`23 Claims, 7 Drawing Sheets
`
`OFDM Symbols to Subscribers
`
`
`
`
`‘Subscriber(s) Continuously Monitors
`Pilot Symbols/Measures SINR and/or
`Other Parameters
`
` Periodically Broadcast Pilot
`
`
`
`Each Subscriber Selects One or More
`Clusters for Each Base Station
`
`
`Base Station Selects Oneor More
`Clustersfor Each Subscriber
`
`
`
`
`-
`Retraining
`Needed2
`
`No
`
`
`
`
`Base Station Notifies the Subscriber
`Regarding ClusterAllocation
`
`1
`
`DELL EX. 1007
`
`DELL EX. 1007
`
`

`

`US 6,947,748 B2
`Page 2
`
`U.S. PATENT DOCUMENTS
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`9/2003 Chayat
`OTHER PUBLICATIONS
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`............ 375/346
`
`Frullone et al., PRMA Performance in Cellular Environ-
`ments with Self—Adaptive Channel Allocation Strategies,
`IEEETransactions on Vehicular Technology, Nov. 1996, pp.
`657-665 vol. 45, No. 4.
`Xu, Guanghan and Li, San—Qi, Throughput Multiplication
`of Wireless Lans for Multimedia Services: SDMAProtocol
`
`Design, 1994 IEEE, pp. 1326-1332.
`Ward, James and Compton, R. Ted, Jr., High Throughput
`Slotted ALOHA Packet Radio Networks with Adaptive
`Arrays, IEEE Transactions on Communications, Mar. 1993,
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`Tsoulos, G.V., Smart antennas for mobile communication
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`
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`Bender, et al. “CDMA/HDR: A Bandwidth Efficient High—
`Speed Wireless Data Service for Nomadic Users,” IEEE
`Communications Magazine, Jul. 2000, pp. 70-87.
`Frullone, et al., “PRMA Performance in Cellular Environ-
`ments with Self—Adaptive Channel Allocation Strategies,”
`IEEETransactions on Vehicular Technology, vol. 45, No. 4,
`Nov. 1996, pp. 657-665.
`Farsakh, C. et al., “Maximizing the SDMA Mobile Radio
`Capacity Increase by DOA Sensitive Channel Allocation”,
`Wireless Personal Communications, Kluwer Academic Pub-
`lishers, NL, vol.
`11, No.
`1, Oct. 1999, pp. 63-76,
`XP000835062, ISSN: 0929-6212.
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`Areas in Communications, Oct. 1999, IEEE Inc., New York,
`USA,vol. 17, Nr. 10, pp. 1747-1758, XP000854075, ISSN:
`0733-8716 Sections I and II abstract.
`
`Gruenheid, R. et al: “Adaptive Modulation and Multiple
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`NL,vol. 13, NR. 1/2, Year 2000, pp. 5-13 XP000894156,
`ISSN: 0929-6212.
`
`Motegi, M. etal.: “Optimum Band Allocation According to
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`XP002213669,
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`NJ,
`USA,
`ISBN:
`0-7803-6463-5.
`
`Kapoor, S. et al.: “Adaptive Interference Suppression in
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`IEEE Transactions on Signal Processing, vol. 47, No. 12,
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`Ye Li, et al.: “Clustered OFDM with channel estimation for
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`
`* cited by examiner
`
`2
`
`

`

`U.S. Patent
`
`Sep. 20, 2005
`
`Sheet 1 of 7
`
`US 6,947,748 B2
`
`Subcarrier
`101
`
`Cluster
`102
`(7
`
`FIG. 1A
`
`Pilot OFDM/7aRRRRR
`Symbols
`SSSSS
`201
`
`
`
`
`{
`
`Occupied Clusters
`
`a. Cell A
`
`t
`
`FIG. 2
`
`c. Cell C
`
`(C)
`
`3
`
`

`

`U.S. Patent
`
`Sep. 20, 2005
`
`Sheet 2 of 7
`
`US 6,947,748 B2
`
`Periodically BroadcastPilot
`OFDM Symbols to Subscribers
`
`104
`
`Subscriber(s) Continuously Monitors
`Pilot Symbols/Measures SINR and/orPY102
`Other Parameters
`
`Retraining
`Needed
`?
`
`Clusters for Each BaseStation
`
`Base Station Selects One or More
`Clusters for Each Subscriber
`
` Each Subscriber Selects One or More
`
`BaseStation Notifies the Subscriber
`Regarding Cluster Allocation
`
`FIG. 1B
`
`108
`
`404
`
`105
`
`4
`
`

`

`U.S. Patent
`
`Sep. 20, 2005
`
`Sheet 3 of 7
`
`US 6,947,748 B2
`
`
`Channel/Interference
`Estimation in Pilot
`
`
`
`Periods
`Cluster Ordering
`Request Selected
`
`and Rate
`Clusters and Coding/
`
`
`
`Modulation Rates
`Prediction
`
`
`
`
`Traffic/interference
`
`Analysis in Date
`Periods
`
`
` Per-cluster SINR
`Estimation in
`
` Cluster Ordering/
`
`Pilot Periods
`404 Selection Based on
`
`
`
`
`Request Selected
`Per-cluster
`SINR and Power
`
`
`Clusters and Coding/
`
`PowerCalculation
`
`
`
`Difference
`
`Modulation Rates
`in Pilot Periods
`
`
`
`
`Per-ciuster
`
`
`PowerCalculation
`
`in Data Periods FIG. 4
`
`501
`
`502
`
`503
`
`504
`
`504
`
`504
`
`Cluster
`
`Cluster
`
`Cluster
`
`coe
`
`FIG. 5
`
`
`
`5
`
`

`

`U.S. Patent
`
`Sep. 20, 2005
`
`Sheet 4 of 7
`
`US 6,947,748 B2
`
`Grow|SINR1|SINR2|SINR3|© © ©
`
`Group
`
`
`
`
`
`
`
`FIG. 7
`
`
`
`
`P
`>
`
`LArbxbrbr
`
`
`
`
`
`
`
`FIG. 8
`
`
`
`6
`
`

`

`U.S. Patent
`
`Sep. 20, 2005
`
`Sheet 5 of 7
`
`US 6,947,748 B2
`
`1-8: Diverse Clusters
`9-16: Plain Clusters
`
`
`
`b. Cell B
`
`
`
`hpHH|fiWu|tt
`ql sa13|14
`
`
`
`
`ie
`qn
`aH{fa|A
`
`
`AE
`Time 4+
`
`f
`
`f
`
`3
`
`
`
`
`
`
`
`
`UNL
`
`
`
`c. Cell C
`
`FIG. 9
`
`it
`
`
`
`
`Hil
`
`
`Subcarrier1
`
`Time 1-8
`Time 2
`Time 3 -RRRR
`
`t
`
`b. Cell B
`
`FIG. 10
`
`7
`
`

`

`U.S. Patent
`
`Sep. 20, 2005
`
`Sheet 6 of 7
`
`US 6,947,748 B2
`
`
`
`
`
`Channel/Interference
`Variation Detection
`
`1101
`
`
`
`
`
`Any
`Significant Variation
`Detected
`?
`
`FIG. 11
`
`
`
`rteFATE:RR
`
`PT STATI
`TTT EP
`
`
`
`
`a. Cell A
`
`FIG. 12
`
`
` i
`fi
`
`
`
`
`
`
`
`
`
`8
`
`

`

`U.S. Patent
`
`Sep. 20, 2005
`
`Sheet 7 of 7
`
`US 6,947,748 B2
`
`User Data Buffer Information
`1311
`
`User 1~N
`
`
`
`
`1302
`
`1303
`
`Multi-user Data
`
`
`Admission Control
`
`
`Cluster Allocation and
`Load Scheduling
`
`Controller
`
`
`
`
`Multi-cluster
`
`Transmission and
`
`
`SINR/Rate
`Receiving Buffer
`
`
`Indices
`
`
`
`Control Signal/
`OFDM Signal
`ClusterAllocation
`
`1312
`
`OFDM Transceiver
`
`1305
`
`FIG. 13
`
`9
`
`

`

`US 6,947,748 B2
`
`1
`OFDMA WITH ADAPTIVE SUBCARRIER-
`CLUSTER CONFIGURATION AND
`SELECTIVE LOADING
`
`FIELD OF THE INVENTION
`
`The invention relates to the field of wireless communi-
`cations; moreparticularly, the invention relates to multi-cell,
`multi-subscriber wireless systems using orthogonal
`fre-
`quency division multiplexing (OFDM).
`BACKGROUND OF THE INVENTION
`
`Orthogonal frequency division multiplexing (OFDM) is
`an efficient modulation scheme for signal transmission over
`frequency-selective channels. In OFDM, a wide bandwidth
`is divided into multiple narrow-band subcarriers, which are
`arranged to be orthogonal with each other. The signals
`modulated on the subcarriers are transmitted in parallel. For
`more information, see Cimini, Jr., “Analysis and Simulation
`of a Digital Mobile Channel Using Orthogonal Frequency
`Division Multiplexing,” IEEE Trans. Commun., vol. COM-
`33, no. 7, July 1985, pp. 665-75; Chuang and Sollenberger,
`“Beyond 3G: Wideband Wireless Data Access Based on
`OFDM and Dynamic Packet Assignment,” IEEE Commu-
`nications Magazine, Vol. 38, No. 7, pp. 78-87, July 2000.
`One way to use OFDM to support multiple access for
`multiple subscribersis through time division multiple access
`(TDMA), in which each subscriber uses all the subcarriers
`within its assigned time slots. Orthogonal frequency division
`multiple access (OFDMA)is another method for multiple
`access, using the basic format of OFDM.
`In OFDMA,
`multiple subscribers simultaneously use different
`subcarriers, in a fashion similar to frequency division mul-
`tiple access (FDMA). For more information, see Sari and
`Karam, “Orthogonal Frequency-Division Multiple Access
`and its Application to CATV Networks,” European Trans-
`actions on Telecommunications, Vol. 9 (6), pp. 507-516,
`November/December 1998 and Nogueroles, Bossert,
`Donder, and Zyablov, “Improved Performance of a Random
`OFDMAMobile Communication System,”, Proceedings of
`IEEE VTC’98, pp. 2502-2506.
`Multipath causes frequency-selective fading. The channel
`gains are different for different subcarriers. Furthermore, the
`channels are typically uncorrelated for different subscribers.
`The subcarriers that are in deep fade for one subscriber may
`provide high channel gains for another subscriber.
`Therefore,
`it
`is advantageous in an OFDMAsystem to
`adaptively allocate the subcarriers to subscribers so that each
`subscriber enjoys a high channel gain. For more
`information, see Wongetal., “Multiuser OFDM with Adap-
`tive Subcarrier, Bit and Power Allocation,” IEEE J. Select.
`Areas Commun., Vol. 17(10), pp. 1747-1758, October 1999.
`Within one cell,
`the subscribers can be coordinated to
`have different subcarriers in OFDMA.Thesignals for dif-
`ferent subscribers can be made orthogonal andthereis little
`intracell interference. However, with aggressive frequency
`reuse plan, e.g.,
`the same spectrum is used for multiple
`neighboring cells,
`the problem of intercell
`interference
`arises.It is clear that the intercell interference in an OFDMA
`system is also frequency selective and it is advantageous to
`adaptively allocate the subcarriers so as to mitigate the effect
`of intercell interference.
`
`One approach to subcarrier allocation for OFDMAis a
`joint optimization operation, not only requiring the activity
`and channel knowledgeofall the subscribersin all the cells,
`but also requiring frequent rescheduling every time an
`
`10
`
`15
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`existing subscribers is dropped off the network or a new
`subscribers is added onto the network. This is often imprac-
`tical in real wireless system, mainly due to the bandwidth
`cost for updating the subscriber information and the com-
`putation cost for the joint optimization.
`
`SUMMARYOF THE INVENTION
`
`A method and apparatus for subcarrier selection for
`systems is described.
`In one embodiment,
`the system
`employs orthogonal frequency division multiple access
`(OFDMA). In one embodiment, a method for subcarrier
`selection comprises a subscriber measuring channel and
`interference information for subcarriers based on pilot sym-
`bols received from a base station, the subscriber selecting a
`set of candidate subcarriers, providing feedback information
`on the set of candidate subcarriers to the base station, and
`receiving an indication of subcarriers of the set of subcar-
`riers selected by the base station for use by the subscriber.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The present invention will be understood morefully from
`the detailed description given below and from the accom-
`panying drawings of various embodiments of the invention,
`which, however, should not be taken to limit the invention
`to the specific embodiments, but are for explanation and
`understanding only.
`FIG. 1A illustrates subcarriers and clusters.
`
`FIG. 1B is a flow diagram of one embodimentof a process
`for allocating subcarriers.
`FIG. 2 illustrates time and frequency grid of OFDM
`symbols, pilots and clusters.
`FIG. 3 illustrates subscriber processing.
`FIG. 4 illustrates one example of FIG. 3.
`FIG. 5 illustrates one embodiment of a format for arbi-
`
`trary cluster feedback.
`FIG. 6 illustrates one embodiment of a partition the
`clusters into groups.
`FIG. 7 illustrates one embodiment of a feedback format
`
`for group-based cluster allocation.
`FIG. 8 illustrates frequency reuse and interference in a
`multi-cell, multi-sector network.
`FIG. 9 illustrates different cluster formats for coherence
`
`clusters and diversity clusters.
`FIG. 10 illustrates diversity clusters with subcarrier hop-
`ping.
`
`FIG. 11illustrates intelligent switching between diversity
`clusters and coherence clusters depending on subscribers
`mobility.
`FIG. 12 illustrates one embodiment of a reconfiguration
`of cluster classification.
`FIG. 13 illustrates one embodimentof a base station.
`
`DETAILED DESCRIPTION OF THE PRESENT
`INVENTION
`
`A distributed, reduced-complexity approach for subcar-
`rier allocation is described. The techniques disclosed herein
`are described using OFDMA (clusters) as an example.
`However, they are not limited to OFDMA-based systems.
`The techniques apply to multi-carrier systems in general,
`where, for example, a carrier can be a cluster in OFDMA,a
`spreading code in CDMA, an antenna beam in SDMA
`(space-division multiple access), etc. In one embodiment,
`subcarrier allocation is performed in each cell separately.
`
`10
`
`10
`
`

`

`US 6,947,748 B2
`
`3
`Within each cell, the allocation for individual subscribers
`(e.g., mobiles) is also made progressively as each new
`subscriber is added to the system as opposed to joint
`allocation for subscribers within each cell in which alloca-
`tion decisions are made taking into accountall subscribers in
`a cell for each allocation.
`
`For downlink channels, each subscriberfirst measures the
`channel and interference information for all the subcarriers
`
`and then selects multiple subcarriers with good performance
`(e.g., a high signal-to-interference plus noise ratio (SINR))
`and feeds back the information on these candidate subcar-
`riers to the base station. The feedback may comprise channel
`and interference information (e.g., signal-to-interference-
`plus-noise-ratio information) on all subcarriers or just a
`portion of subcarriers. In case of providing information on
`only a portion of the subcarriers, a subscriber may provide
`a list of subcarriers ordered starting with those subcarriers
`which the subscriber desires to use, usually because their
`performanceis good or better than that of other subcarriers.
`Upon receiving the information from the subscriber, the
`base station further selects the subcarriers among the
`candidates, utilizing additional information available at the
`base station, e.g.,
`the traffic load information on each
`subcarrier, amount of traffic requests queued at the base
`station for each frequency band, whether frequency bands
`are overused, and/or how long a subscriber has been waiting
`to send information. In one embodiment,
`the subcarrier
`loading information of neighboring cells can also be
`exchanged betweenbase stations. The base stations can use
`this information in subcarrier allocation to reduce inter-cell
`interference.
`
`In one embodiment, the selection by the base station of
`the channels to allocate, based on the feedback,results in the
`selection of coding/modulation rates. Such coding/
`modulation rates may be specified by the subscriber when
`specifying subcarriers that it finds favorable to use. For
`example, if the SINRis less than a certain threshold(e.g., 12
`dB), quadrature phase shift keying (QPSK) modulation is
`used; otherwise, 16 quadrature amplitude modulation
`(QAM)is used. Then the base station informs the subscrib-
`ers about
`the subcarrier allocation and the coding/
`modulation rates to use.
`
`In one embodiment, the feedback information for down-
`link subcarrier allocation is transmitted to the base station
`
`through the uplink access channel, which occurs in a short
`period every transmission timeslot, e.g., 400 microseconds
`in every 10-millisecond time slot. In one embodiment, the
`access channel occupies the entire frequency bandwidth.
`Then the base station can collect the uplink SINR of each
`subcarrier directly from the access channel. The SINR as
`well as thetraffic load information on the uplink subcarriers
`are used for uplink subcarrier allocation.
`For either direction,
`the base station makes the final
`decision of subcarrier allocation for each subscriber.
`
`In the following description, a procedure of selective
`subcarrier allocation is also disclosed, including methods of
`channel and interference sensing, methods of information
`feedback from the subscribers to the base station, and
`algorithmsused bythe basestation for subcarrier selections.
`In the following description, numerous details are set
`forth to provide a thorough understanding of the present
`invention. It will be apparent, however, to one skilled in the
`art, that the present invention may bepracticed without these
`specific details. In other instances, well-known structures
`and devices are shown in block diagram form,rather than in
`detail, in order to avoid obscuring the present invention.
`
`4
`Someportions of the detailed descriptions which follow
`are presented in terms of algorithms and symbolic repre-
`sentations of operations on data bits within a computer
`memory. These algorithmic descriptions and representations
`are the means used by those skilled in the data processing
`arts to most effectively convey the substance of their work
`to others skilled in the art. An algorithm is here, and
`generally, conceived to be a self-consistent sequence of steps
`leading to a desired result. The steps are those requiring
`physical manipulations of physical quantities. Usually,
`though not necessarily, these quantities take the form of
`electrical or magnetic signals capable of being stored,
`transferred, combined, compared, and otherwise manipu-
`lated. It has proven convenient at times, principally for
`reasons of common usage, to refer to these signals as bits,
`values, elements, symbols, characters, terms, numbers, or
`the like.
`
`It should be borne in mind, however,that all of these and
`similar terms are to be associated with the appropriate
`physical quantities and are merely convenient labels applied
`to these quantities. Unless specifically stated otherwise as
`apparent from the following discussion,it is appreciated that
`throughout the description, discussions utilizing terms such
`as “processing” or “computing” or “calculating” or “deter-
`mining” or “displaying” or the like, refer to the action and
`processes of a computer system, or similar electronic com-
`puting device, that manipulates and transforms data repre-
`sented as physical (electronic) quantities within the com-
`puter system’s registers and memories into other data
`similarly represented as physical quantities within the com-
`puter system memories or registers or other such informa-
`tion storage, transmission or display devices.
`The present invention also relates to apparatus for per-
`forming the operations herein. This apparatus may be spe-
`cially constructed for the required purposes, or it may
`comprise a general purpose computer selectively activated
`or reconfigured by a computer program stored in the com-
`puter. Such a computer program maybestored in a computer
`readable storage medium,suchas, but is not limited to, any
`type of disk including floppy disks, optical disks,
`CD-ROMs,and magnetic-optical disks, read-only memories
`(ROMs),
`random access memories (RAMs), EPROMs,
`EEPROMs, magnetic or optical cards, or any type of media
`suitable for storing electronic instructions, and each coupled
`to a computer system bus.
`The algorithms and displays presented herein are not
`inherently related to any particular computer or other appa-
`ratus. Various general purpose systems may be used with
`programsin accordance with the teachings herein, or it may
`prove convenient to construct more specialized apparatus to
`perform the required method steps. The required structure
`for a variety of these systems will appear from the descrip-
`tion below.
`In addition,
`the present
`invention is not
`described with reference to any particular programming
`language. It will be appreciated that a variety of program-
`ming languages may be used to implementthe teachings of
`the invention as described herein.
`
`A machine-readable medium includes any mechanism for
`storing or transmitting information in a form readable by a
`machine (e.g., a computer). For example, a machine-
`readable medium includes read only memory (“ROM”);
`random access memory (“RAM”); magnetic disk storage
`media; optical storage media;
`flash memory devices;
`electrical, optical, acoustical or other form of propagated
`signals (e.g., carrier waves, infrared signals, digital signals,
`etc.); etc.
`
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`

`

`US 6,947,748 B2
`
`5
`
`Subcarrier Clustering
`The techniques described herein are directed to subcarrier
`allocation for data traffic channels.In a cellular system, there
`are typically other channels, pre-allocated for the exchange
`of control information and other purposes. These channels
`often include downlink and up link control channels, uplink
`access channels, and time and frequency synchronization
`channels.
`FIG. 1A illustrates multiple subcarriers, such as subcarrier
`101, and cluster 102. Acluster, such as cluster 102, is defined
`as a logical unit that contains at least one physical subcarrier,
`as shown in FIG. 1A. A cluster can contain consecutive or
`
`disjoint subcarriers. The mapping between a cluster and its
`subcarriers can be fixed or reconfigurable. In the latter case,
`the base station informsthe subscribers whentheclusters are
`redefined.
`In one embodiment,
`the frequency spectrum
`includes 512 subcarriers and each cluster includes four
`
`consecutive subcarriers, thereby resulting in 128 clusters.
`An Exemplary Subcarrier/Cluster Allocation Procedure
`FIG. 1B isa flow diagram of one embodimentof a process
`for allocation clusters to subscribers. The process is per-
`formed by processing logic that may comprise hardware
`(e.g., dedicated logic, circuitry, etc.), software (suchas that
`which runs on, for example, a general purpose computer
`system or dedicated machine), or a combination of both.
`Referring to FIG. 1B, each base station periodically
`broadcasts pilot OFDM symbols to every subscriber within
`its cell (or sector) (processing block 101). The pilot symbols,
`often referred to as a sounding sequence or signal, are
`knownto both the base station and the subscribers. In one
`
`embodiment, each pilot symbol covers the entire OFDM
`frequency bandwidth. The pilot symbols maybe different for
`different cells (or sectors). The pilot symbols can serve
`multiple purposes:
`time and frequency synchronization,
`channel estimation and signal-to-interference/noise (SINR)
`ratio measurementfor cluster allocation.
`
`Next, each subscriber continuously monitors the reception
`of the pilot symbols and measures the SINR and/or other
`parameters, including inter-cell interference and intra-cell
`traffic, of each cluster (processing block 102). Based onthis
`information, each subscriber selects one or more clusters
`with good performance (e.g., high SINR and lowtraffic
`loading) relative to each other and feeds back the informa-
`tion on these candidate clusters to the base station through
`predefined uplink access channels (processing block 103).
`For example, SINR values higher than 10 dB may indicate
`good performance. Likewise, a cluster utilization factor less
`than 50% may be indicative of good performance. Each
`subscriber selects the clusters with relatively better perfor-
`mance than others. The selection results in each subscriber
`
`selecting clusters they would prefer to use based on the
`measured parameters.
`In one embodiment, each subscriber measures the SINR
`of each subcarrier cluster and reports these SINR measure-
`ments to their base station through an access channel. The
`SINR value may comprise the average of the SINR values
`of each of the subcarriers in the cluster. Alternatively, the
`SINR value for the cluster may be the worst SINR among
`the SINR values of the subcarriers in the cluster. In still
`
`another embodiment, a weighted averaging of SINR values
`of the subcarriers in the cluster is used to generate an SINR
`value for the cluster. This may beparticularly useful in
`diversity clusters where the weighting applied to the sub-
`carriers may be different.
`The feedback of information from each subscriber to the
`base station contains a SINR value for each cluster and also
`
`indicates the coding/modulation rate that
`
`the subscriber
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`desires to use. No cluster index is needed to indicate which
`SINR value in the feedback corresponds to which cluster as
`long as the order of information in the feedback is known to
`the base station. In an alternative embodiment, the informa-
`tion in the feedback is ordered according to which clusters
`have the best performance relative to each other for the
`subscriber. In such a case, an index is neededto indicate to
`which cluster the accompanying SINR value corresponds.
`Uponreceiving the feedback from a subscriber, the base
`station further selects one or moreclusters for the subscriber
`among the candidates (processing block 104). The base
`station may utilize additional information available at the
`base station, e.g.,
`the traffic load information on each
`subcarrier, amount of traffic requests queued at the base
`station for each frequency band, whether frequency bands
`are overused, and how long a subscriber has been waiting to
`send information. The subcarrier loading information of
`neighboring cells can also be exchanged between base
`stations. The base stations can use this information in
`subcarrier allocation to reduce inter-cell interference.
`
`After cluster selection, the base station notifies the sub-
`scriber about
`the cluster allocation through a downlink
`common control channel or through a dedicated downlink
`traffic channel if the connection to the subscriber has already
`been established (processing block 105).
`In one
`embodiment, the base station also informs the subscriber
`about the appropriate modulation/codingrates.
`Once the basic communication link is established, each
`subscriber can continue to send the feedback to the base
`station using a dedicated traffic channel (e.g., one or more
`predefined uplink access channels).
`In one embodiment,
`the base station allocates all the
`clusters to be used by a subscriber at once. In an alternative
`embodiment,the basestationfirst allocates multiple clusters,
`referred to herein as the basic clusters, to establish a data link
`between the base station and the subscriber. The base station
`
`then subsequently allocates more clusters, referred to herein
`as the auxiliary clusters, to the subscriber to increase the
`communication bandwidth. Higherpriorities can be given to
`the assignmentof basic clusters and lowerpriorities may be
`given to that of auxiliary clusters. For example, the base
`station first ensures the assignment of the basic clusters to
`the subscribers and then tries to satisfy further requests on
`the auxiliary clusters from the subscribers. Alternatively, the
`base station may assign auxiliary clusters to one or more
`subscribers before allocating basic clusters to other subscrib-
`ers. For example, a base station may allocate basic and
`auxiliary clusters to one subscriber before allocating any
`clusters to other subscribers. In one embodiment, the base
`station allocates basic clusters to a new subscriber and then
`
`determines if there are any other subscribers requesting
`clusters. If not, then the base station allocates the auxiliary
`clusters to that new subscriber.
`
`From time to time, processing logic performsretraining
`by repeating the process described above (processing block
`106). The retraining may be performed periodically. This
`retraining compensates for subscriber movement and any
`changes in interference. In one embodiment, each subscriber
`reports to the base station its updated selection of clusters
`and their associated SINRs. Then the base station further
`
`performs the reselection and informs the subscriber about
`the new cluster allocation. Retraining can be initiated by the
`base station, and in which case, the base station requests a
`specific subscriber to report its updated cluster selection.
`Retraining can also be initiated by the subscriber when it
`observes channel deterioration.
`
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`

`

`US 6,947,748 B2
`
`7
`Adaptive Modulation and Coding
`In one embodiment, different modulation and codingrates
`are used to support reliable transmission over channels with
`different SINR. Signal spreading over multiple subcarriers
`may also be used to improve the reliability at very low
`SINR.
`An example coding/modulation table is given below in
`Table 1.
`
`TABLE1
`
`Scheme
`
`Modulation
`
`Code Rate
`
`DAMPWNHrCO
`
`QPSK, % Spreading
`QPSK, “4 Spreading
`QPSK, 2 Spreading
`QPSK
`8PSK
`16QAM
`64QAM
`
`In the example above, ¥ spreading indicates that one
`QPSK modulation symbolis repeated over eight subcarriers.
`The repetition/spreading may also be extended to the time
`domain. For example, one QPSK symbol can be repeated
`over four subcarriers of two OFDM symbols, resul