`(10) Patent N0.:
`US 6,904,283 132
`
`Li et al.
`(45) Date of Patent:
`Jun. 7, 2005
`
`USOO6904283B2
`
`(54) MULTI-CARRIER COMMUNICATIONS
`WITH GROUP-BASED SUBCARRIER
`ALLOCATION
`
`(75)
`
`Inventors: Xiaodong Li, Bellevue, WA (US); Hui
`Liu, Sammamish, WA (US); Hujun
`Yin, Seattle, WA (US); Guanbin Xing,
`Bellevue, WA (us); Fuqi Mu,
`Issaquah> WA (Us)
`
`(73) Assignee: Adaptix, IIIC., BOthell, WA (US)
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 727 days.
`
`.
`(21) Appl‘ No" 09/837’337
`(22)
`Filed:
`Apr. 17, 2001
`
`(65)
`
`Prior Publication Data
`US 2003/0169681 A1 Sep. 11 2003
`7
`Related US. Application Data
`
`(63)
`(51)
`
`(52) U S Cl
`
`ggntipgatzigrégn—part of application No. 09/738,086, filed on
`Int. Cl.7 ............................ H04B 7/00- H04B 1/38'
`H04Q 7/20. H04M 1/00’. H04] 11/06
`455/450. 455/69 455/447.
`455/448; 455/561; 455/5501, 370/208
`(58) Field of Search ................................. 375/133, 135,
`375/260, 267; 370/203, 208, 210, 482,
`484, 431, 319_321’ 328, 329, 332, 342,
`344’ 345; 455/69, 434’ 455’ 463, 4221,
`447_450’ 452.1, 452.2, 453, 42, 500, 512,
`513, 102_105
`
`(56)
`
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`Primary Examiner—William Trost
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`(74) Attorney, Agent, or Firm—Blakely, Sokoloff, Taylor &
`Zafma“ LLP
`(57)
`
`ABSTRACT
`.
`.
`Amethod and apparatus for subcarrier selection for systems
`is described. In one embodiment, a method for subcarrier
`selection for a system employing orthogonal frequency
`division multiple access (OFDMA) comprises partitioning
`subcarriers into groups of at least one cluster of subcarriers,
`receiving an indication of a selection by the subscriber of
`one or more groups in the groups, and allocating at least one
`cluster in the one or more groups of clusters selected by the
`subcarrier for use in communication with the subscriber.
`
`119 Claims, 7 Drawing Sheets
`
`
`
`Retraining
`Needed
`
`7
`
`
`
`
`Parlcdically Broadcast Pilot
`OFDM Symbols to Subscribers
`
`Subscriber(s) Continuously Monitors
`Pllot Symbols/Measures SINR and/or
`Other Parameters
`
`Each Subscriber Selects One or More
`Clusters tor Each Base Station
`
`Base Station Selects One or More
`Clusters for Each Subscriber
`
`
`
`
`Base Station Notifies the Suhscriber
`Regarding Cluster Allocation
`
`
`
`101
`
`102
`
`103
`
`104
`
`105
`
`|PR2019-00958
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`US 6,904,283 132
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`Page 2
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`
`* cited by examiner
`
`|PR2019-00958
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`US. Patent
`
`Jun. 7, 2005
`
`Sheet 1 0f 7
`
`US 6,904,283 132
`
`Subcarrier
`101
`
`>
`
`Cluster
`K102
`
`FIG. 1A
`
`f
`
`Cluster A
`
`
`
`Cluster B
`
`
`
`________.
`
`Pilot OFDM ——-————
`
`Symbols
`201
`
`t
`
`Occupied Clusters
`
`a. Cell A
`
`MW
`
`201
`”(3):
`\————..............
`
`
`
`201
`
`\IIIIIRII\\
`
`t
`
`FIG. 2
`
`c. Cell C
`(C)
`
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`
`Jun. 7, 2005
`
`Sheet 2 0f 7
`
`US 6,904,283 132
`
`Periodically Broadcast Pilot
`OFDM Symbols to Subscribers
`
`101
`
`'
`
`Subscriber(s) Continuously Monitors
`Pilot Symbols/Measures SINR and/or
`Other Parameters
`
`102
`
`Each Subscriber Selects One or More
`Clusters for Each Base Station
`
`103
`
`Retraining
`Needed
`
`?
`
`104
`
`105
`
`
`
`Base Station Selects One or More
`Clusters for Each Subscriber
`
`Base Station Notifies the Subscriber
`
`Regarding Cluster Allocation
`
`FIG. 18
`
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`Sheet 3 0f 7
`
`US 6,904,283 132
`
`Periods
`
`
`Channel/lnterference
`Estimation in Pilot
`
`
`
`
`
`Request Selected
`Cluster Ordering
`
`
`and Rate
`Clusters and Coding!
`
`
`
`Modulation Rates
`Prediction
` Traffic/Interference
`
`
`Analysis in Date
`Periods
`
`
`
`Per-cluster SINR
`401
`
`Estimation in
`405
`
`
`Pi'Ot Periods
`Cluster Ordering/
`405
`
`,
`Selection Based on
`
`
`
`
`Per-cluster
`SINR and Power
`Request Selected
`Power Calculation
`Clusters and Coding/
`
`
`
`
`leference
`Modulation Rates
`in Pilot Periods
`
`
`
`
`Per-cluster
`Power Calculation
`
`in Data Periods
`403
`
`
`404
`
`FIG. 4
`
`501
`
`502
`
`503
`
`504
`
`504
`
`504
`
`Cluster
`
`Cluster
`
`Cluster
`
`Group 3
`
`Group 4
`
`
`
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`Jun. 7, 2005
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`Sheet 4 0f 7
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`US 6,904,283 132
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`
`
`
`
`
`
`
`
`
`\\\\V\\§\V€\§V\%V\\§\
`
`
`mgmv
`
`FIG. 8
`
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`Sheet 5 0f 7
`
`US 6,904,283 132
`
`1-8: Diverse Clusters
`946: Plain Clusters
`
`f
`
`
`
`
`w lllllllllll w Illllllll
`”mull”
`lIlI ll
`I
`
`
`
`
`
`
`llllllllllllll'llllllllll lll
`Illll
`IIIIIl
`I
`III
`
`
`Hgll:II
`
`a. CellA
`
`
`
`
`
`b. Cell B
`
`t
`
`t
`
`t
`
`Subcarrier 1
`
`
`
`
`--§33$""--3333 - ~IIII -
`Time1§§§
`--!§s§||||--s§§ - nu -
`Timez
`--sg§gIIII--§g§s - all" I
`““183 .§§§
`
`
`Time4 m --S&§sllll--;€s:§s - {émllll -
`l
`a.CeI|A
`
`a
`
`--
`-
`-— I
`
`-
`-
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`-:v'r.'.-'...o''_-_
`'V’JI.'
`
`r:
`“ III--I ‘
`3th
`l-
`
`
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`Sheet 6 0f 7
`
`US 6,904,283 132
`
`Channel/Interference
`
`1101
`
`Variation Detection
`
`
`
`
`
`1102
`
`
`
`
`Yes
`
` Any
`Significant Variation
`Detected
`
`
`1104
`
`1103
`
`?
`
`Select Diversity
`Clusters
`
`Select Coherence
`
`Clusters
`
`FIG. 11
`
`
`
`Illllillli
`I
`. n i.
`ll
`.
`’*'llllll“llllllllllll‘llil'llllll1"lllllllfl
`
`a. Cell A
`
`FIG. 12
`
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`Sheet 7 0f 7
`
`US 6,904,283 132
`
`User Data Buffer Information
`1311
`
`User1~ N
`
`Cluster Allocation and
`
`Multi-user Data
`
`Buffer
`
`Multiplexer
`
`
`
`
`
`Admission Control
`1301
`1310
`
`
`Load Scheduling
`
`
`
`Controller
`
`
`
`
`
`Multi-cluster
`
`Transmission and
`
`
`
`
`
`Cluster 1 ~ M
`
`1302
`
`1 303
`
`1 304
`
`1305
`
`SINR/Rate
`
`lndices
`
`Receiving Buffer
`
`1313
`
`
`
`OFDM Transceiver
`
`Control Signal/
`Cluster Allocation
`
`1312
`
`OFDM Signal
`
`
`
`FIG. 13
`
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`US 6,904,283 B2
`
`1
`MULTI-CARRIER COMMUNICATIONS
`WITH GROUP-BASED SUBCARRIER
`ALLOCATION
`
`This patent application is a Continuation-in-part (CIP) of
`patent application Ser. No. 09/738,086 filed Dec. 15, 2000,
`entitled “OFDMA with Adaptive Subcarrier-Cluster Con-
`figuration and Selective Loading.”
`
`FIELD OF THE INVENTION
`
`The invention relates to the field of wireless communi-
`
`cations; more particularly, 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 subscribers is 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
`OFDMA Mobile 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 OFDMA system to
`adaptively allocate the subcarriers to subscribers so that each
`subscriber enjoys a high channel gain. For more
`information, see Wong et al., “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. The signals for dif-
`ferent subscribers can be made orthogonal and there is 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.
`
`2
`One approach to subcarrier allocation for OFDMA is a
`joint optimization operation, not only requiring the activity
`and channel knowledge of all the subscribers in all the cells,
`but also requiring frequent rescheduling every time an
`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.
`
`SUMMARY OF THE INVENTION
`
`A method and apparatus for subcarrier selection for
`systems is described. In one embodiment, a method for
`subcarrier selection for a system employing orthogonal
`frequency division multiple access (OFDMA) comprises
`partitioning subcarriers into groups of at least one cluster of
`subcarriers, receiving an indication of a selection by the
`subscriber of one or more groups in the groups, and allo-
`cating at least one cluster in the one or more groups of
`clusters selected by the subcarrier for use in communication
`with the subscriber.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The present invention will be understood more fully 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 embodiment of 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. 11 illustrates 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 embodiment of a base station.
`
`DETAILED DESCRIPTION OF THE PRESENT
`INVENTION
`
`An approach for subcarrier allocation is described. A
`method and apparatus for subcarrier selection for systems is
`described.
`In one embodiment, a method for subcarrier
`selection for a system employing orthogonal frequency
`division multiple access (OFDMA) comprises partitioning
`subcarriers into groups of at least one cluster of subcarriers,
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`3
`receiving an indication of a selection by the subscriber of
`one or more groups in the groups, and allocating at least one
`cluster in the one or more groups of clusters selected by the
`subcarrier for use in communication with the subscriber.
`
`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. 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 allocation decisions are made taking into
`account all subscribers in a cell for each allocation.
`
`For downlink channels, each subscriber first 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
`performance is 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 between base 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 SINR is 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.
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`In one embodiment, the feedback information for down-
`link subcarrier allocation is transmitted to the base station
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`through the uplink access channel, which occurs in a short
`period every transmission time slot, 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 the traffic 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.
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`In the following description, a procedure of selective
`subcarrier allocation is also disclosed, including methods of
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`channel and interference sensing, methods of information
`feedback from the subscribers to the base station, and
`algorithms used by the base station 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 be practiced 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.
`Some portions 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 “rprocessing” 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 may be stored in a computer
`readable storage medium, such as, 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
`programs in 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 implement the teachings of
`the invention as described herein.
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`5
`Amachine-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.
`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 down link 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 informs the subscribers when the clusters are
`
`the frequency spectrum
`In one embodiment,
`redefined.
`includes 512 subcarriers and each cluster includes four
`
`consecutive subcarriers, thereby resulting in 128 clusters.
`An Exemplary Subcarrier/Cluster Allocation Procedure
`FIG. 1B is a flow diagram of one embodiment of 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 (such as 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
`known to both the base station and the subscribers. In one
`
`embodiment, each pilot symbol covers the entire OFDM
`frequency bandwidth. The pilot symbols may be 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 measurement for 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 on this
`information, each subscriber selects one or more clusters
`with good performance (e.g., high SINR and low traffic
`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
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`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 be particularly 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
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`the subscriber
`indicates the coding/modulation rate that
`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 needed to indicate to
`which cluster the accompanying SINR value corresponds.
`Upon receiving the feedback from a subscriber, the base
`station further selects one or more clusters 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/coding rates.
`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 base station first 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 in