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`68144/P014C1/10503148
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`OFDMA WITH ADAPTIVE SUBCARRlER—
`CLUSTER CONFIGURATION AND SELECTIVE
`LOADING
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`. UNITED STATES PATENT APPLICATION
`
`'
`
`~
`
`for
`
`_ OFVDMAWITI-I “ADAPTIVE SUBCARRIER-CLUSTER
`CONFIGURATION AND SELECTIVE LOADING I
`
`Inventors:
`
`Xiaodong Li
`Hui Liu
`
`Kemin Li
`Wenzhong Zhang
`
`BESTIAVAILABLE copv
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`Page 3
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`

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`Page 4
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`OFDMA WITH;ADAPTIVE SUBCARRIER-CLUSTER
`CONFIGURATION AND SELECTIVE LOADING
`
`2
`
`FIELD OF THE INVENTION ‘
`
`The invention relates to the field of wireless communications; more
`particularly, the invention relates to Inulti-cell, multi—subscriber wireless
`
`systems using orthogonal frequency division multiplexing (OFDM).
`
`BACKGROUND OF THE INVENTION A
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`10
`
`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
`
`15
`
`information, see Cimini, Ir., ”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 Assigmnent,” IEEE Communications Magazine, Vol.
`
`20
`
`38, No. 7, pp. 78-87, July 2000.
`
`‘ One way to use to support multiple access formultiple I
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`subscribers is through time division multiple access (TDMA), in which each
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`Page 5
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`V 3
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`subscriber uses all the subcarriers within its assigned time slots. Onhogonal
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`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 multiple access (FDMA). For more information, see Sari and
`
`Karam, ”Orthogonal Frequency—Division Multiple Access and its
`
`Application to CATV Networks," European Transactions on
`
`Telecommunications, Vol. 9- (6), pp. 507-516, Nov./Dec. 1998 and
`
`Nogueroles, Bossert, Donder, and Zyablov, ’’Improved Performance of a
`
`10 Random OFDMA Mobile Communication System/’, Proceedings of IEEE ’
`
`VTC’98, pp. 250; -2505.
`Multipath causes frecl1uency¥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 highchannel gains for another subscriber.
`
`15
`
`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 moreinfonnation, see Wong et a1., ”Multiuser OFDM with
`
`Page 5
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`Page 6
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`4
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`Adaptive 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 different 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 adaptivelyallocate the subcarriers so as to mitigate
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`10
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`the effect of intercell interference.
`
`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 isidropped offithe network
`
`.15
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`' or a new subscribers is added onto the network.
`
`is often impractical in
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`real wireless. system, mainly due to the bandwidth cost for updating the '
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`subscriber information and the computation cost for the joint optimization.
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`Page 7
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`SUMMARY OF 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
`symbols 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
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`10
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`subcarriers of
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`set of subcarriers selected by the base station for use by
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`the subscriber.
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`Page 7
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`Page 8
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`' 6
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`_ The present invention will be understood more fully from the
`detailed description given below and from the accompanying 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 V
`
`5
`
`and understanding only.
`
`Figure 1A illustrates subcarriers and clusters.
`
`10
`
`Figure
`
`is a flow diagram of one embodiment of a process for
`
`— allocating subcarriers.
`
`Figure 2 illustrates time and frequency grid of OFDM symbols, pilots
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`and clusters.
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`15 .
`
`F
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`.
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`Figure 3 illustrates subscriber processing.
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`Figure 4 illustrates one ekample of Figure 3'.
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`20
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`Figure 5 illustrates one embodiment of a format for arbitrary cluster
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`feedback.
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`Page 9
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`7
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`Figure 6 iliustrates one embodiment of a partition the clusters into
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`groups.
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`Figure 7 illustrates one embodiment of a feedback format for group-
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`5
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`based cluster allocation.
`
`A Figure 8 illustrates frequency reuse
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`interference in a multi—cell,
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`multi-sector network.
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`10
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`Figure 9 illustrates different cluster formats for coherence clusters
`and diversity clusters.
`
`Figure 10 illustrates diversity clusters with subcarrier hopping.
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`15
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`Figure 11 illustrates intelligent switching between diversity clusters
`and coherence clusters depending on subscribers mobility.
`
`Figure 12 illustrates one embodiment of a reconfiguration of cluster
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`classification.
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`'20
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`Figure 13 illustrates one embodiment of a base station.’
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`Page 10
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`DETAILED DEECRIPTION OF THE PRESENT INVENTION
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`8
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`A distributed, reduced-complexity approach for subcarrier allocation
`' is described.
`techniques disclosed herein are described using OFDMA
`
`I
`
`(clusters) as an eicample. However, they are not limited to OFDMA-based V
`
`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 inSDMA (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
`
`10.
`
`also made progressively as each new subscriber is added to the system as
`
`opposed to jointzallocation for subscribers within each cell in which
`
`allocation decisions are made taking into account all subscribers in a cell for A
`
`each allocation.
`
`For downlink channels, each subscriberifirst measures the channel
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`15
`
`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 (SINll)) and feeds back the information on these candidate
`
`subcarriers to the base station. The feedback may comprise channel and
`
`. interference infcirmation (e.g., signal-to-interference-plus-noise—ratio
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`Page 10
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`

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`Page ll
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`9
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`information) on all subcarriers or just a portion of subcarriers. In case of
`providing inforrnation on only a portion of the subcarriers, a subscriber may _
`provide a list of_;subcarriers ordered starting with those subcarriers which
`the subscriberdesires 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 subcarrierjamount of traffic requests queued at the base station for
`
`10
`
`each frequency band, whether frequency bands are overused, and /or how
`long a subscriber has been waiting to send information. In one embodirnent,
`
`the subcarrier loading information of neighboring cells can also be
`exchanged between base stations; The baselstations can use this information
`in subcarrier allocation to reduce inter-cell interference. I
`In one ernbodiment, the selection by the base station of the channels
`
`15
`
`to allocate, based on the feedback, results in the selection of
`coding/modulation rates. Such coding/modulation rates rnay be specified
`by the subscriber when specifying subcarriers that it finds favorable to use.
`For example, if
`SINR is less than a certain threshold (e.g., _12 dB),
`
`s
`
`_.....9va:\,~»-v.-..
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`Page 11
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`Page 12
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`1 O
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`quadrature bhase shift keying (QPSK) modulation is used; otherwise, 16
`
`quadrature amplitude modulation (QAM) is used. Then the base station
`
`informs the subscribers about the subcarrier allocation and the
`
`"coding/ modulation rates to use.
`
`In one embodiment, the feedback information for downlink
`subcarrier allocation is transmitted to the base station through the uplink '
`access channel, occurs in a shortperiod every transmission time slot,
`e.g., 400 microseéconds in eirery 10-millisecond time slot.
`one
`embodiment,
`access channel occupies the entire frequency bandwidth.
`Then the base stgltion can collect the up1inl< SINR of each subcarrier directly
`from the access channel. The SINR as‘ well as the traffic load information on
`
`10
`
`the up1inl< subcarriers are used for -uplink subcarrier allocation.
`
`For either direction, the base station makes the final decision of
`
`. subcarrier allocation for each subscriber.
`
`15
`
`In the following description, a" procedure of selective subcarrier I
`
`allocation is also disclosed, including methods of channeland interference
`
`sensing, methods of information feedback from the subscribers to the base
`
`station, and algc;-rithrns used by the base station for subcarrier selections.
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`Page 12
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`Page 13
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`1 1
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`‘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;sl<il1ed 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. V
`
`Some portions of the detailed descriptions which follow are presented
`in terms of algorithms and symbolic representations of operations on data
`
`10
`
`bits within aiconirputer memory. These algorithmic descriptions and
`representations are the means used by those skilled in the data processing _
`arts to most effeiictively convey the substance of their work to others skilled
`in the art. An algorithm is here, and generally, conceived to be a self-'
`
`consistent sequeimce of steps leading to a desired result. The steps are those
`
`' requiring physical manipulations of physical quantities. Usually, though
`
`,15
`
`not necessarily, these quantities take the form of electrical or magnetic
`
`signals capable of being stored, transferred, combined, compared, and
`
`otherwise manipulated. It has proven convenient at times, principally for .
`
`reasons of common usage, to refer to these signals as bits, v_a1ues, elements,
`
`symbols, characters, terms, numbers, or the like.
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`Page 14
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`12
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`It should The 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 "determining" or
`
`"displaying" or the like, refer to the action and processes of a computer
`
`system, or similar electronic computing ‘device, that manipulates and
`transforms data‘;represented as physical (electronic) quantities within the
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`10
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`computer system's registers and memories into other data similarly
`
`represented as physical quantities within the computer system memories or
`
`registers or other such information storage, transmission or display devices.
`
`The present invention also relates to apparatus for performing the
`
`operations herein This apparatus‘ may begspecially constructed for the
`
`15
`
`required purposes, or it may comprise a general purpose computer
`
`’ selectively activalted or reconfigured by a computer program stored in the
`
`computer. Such a computer program may be stored in a computer readable
`
`storage mediurrf, such as, but is not limited to, any type of disk including
`
`floppy disks, optical disks, CD-ROMS, and magnetic-optical disks, read-only
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`Page 15
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`1 3
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`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 apparatus. 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 description below. In
`addition, the present invention is not described with reference to any
`particular programming language. It will be appreciated that a variety of
`programming languages may be used to implement the 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
`
`if
`2
`
`-
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`10
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`15
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`Page 15
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`Page 16
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`_
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`other form of propagated signals (e.g., carrier waves, infrared signals, digital .
`
`14
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`signals, etc.); etc.
`
`Subcarrier Clustering
`
`The techniques described herein are directed to subcarrier allocation
`
`for data traffic
`
`In a cellular system, there are typically other
`
`channels, pre-allocated for the exchange of control information and other
`
`burposes. These; channels often include down link and up link control
`channels, uplink: access channels, and time and frequency synchronization
`
`10
`
`channels.
`
`V
`
`Figure
`
`illustrates multiple subcarriers, such as subcarrier 101, and
`
`cluster 102. A cluster, such as cluster 102, is defined as a logical unit that
`
`» contains at leastfone physical subcarrier, as shown in Figure 1A. A cluster
`
`can contain consecutive or disjoint subcarriers. The mapping between a
`
`15
`
`cluster and its subcarriers can be fixed or reconfigurable. In the latter case,
`
`the base station informs the subscribers when the clusters are redefined. In
`
`one embodiment, the frequency spectrum includes 512 subcarriers and each
`
`cluster includes "four consecutive subcarriers, thereby resulting in 128
`
`clusters.
`
`Page 16
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`

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`Page 17
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`15
`
`An'Exempla_ijy Subcarrier [ Cluster Allocation Procedure
`Figure 1B: is a flow diagram of one embodiment of a process for
`allocation clusters tosubscribers. The process is performed 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 combinationof both.
`Referring;to Figure 1B, each base station periodically broadcasts pilot
`OFDM symbolslito every subscriber vtrithin its cell (or sector) (processing
`. block 101). The lpilotsymbols, often referred to as a sounding sequence or
`
`i
`
`5
`
`10 ’
`
`signal, are knowm 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 differentcells (or
`‘ sectors). The pilot symbols can serve multiple purposes: time
`frequency
`rence/noise (SINR)
`
`synchronization‘; channel estimation and signal~to-interfe
`
`' 15
`
`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 mterferenceéand intra-cell traffic, of each cluster (processing block» 102).
`
`Page 17
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`Page 18
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`1 6
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`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 information on these candidate clusters to the base
`
`‘station through predefined uplink access channels (processing block 103).
`
`For example, SIl\lR 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 :performance than others. The selection results
`each
`
`subscriber selecting clusters they would prefer to use based on the measured
`
`10
`
`parameters.
`
`In one ernbodirnent, each subscriber measures the SINR of each
`subcarrier clusteir and reports these SINR measurements 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
`
`15
`
`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 be particularly useful in diversity
`
`clusters where the weighting applied to the subcarriers may be different.
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`Page 18
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`Page 19
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`17
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`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 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
`
`the feedback is known to
`
`the base station. ’ In an alternative embodiment, the information in the
`feedback is ordered according to which clusters have the best performance
`relative to each other for the subscriber. Insuch a case, an index is needed to
`indicate to
`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
`
`10
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`available at the base station, e.g., the traffic load information on each
`
`subcarrier, amount of traffic requests queued at the base station foreach
`frequency bandjwhether frequency bands are overused, and how long a
`
`.15
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`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
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`reduce inter-cell: interference.
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`1 8
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`After cluster selection, the base station notifies the subscriber 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 rnodulation/ 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 ernbodirnent, the base station allocates all the clusters to be
`
`10
`
`used by a subscriber at once. In an alternative embodiment, the base station
`first allocates miilfiple clusters, referred to herein as the basic clusters, to
`
`between the base station and the subscriber. The base
`establish a data
`station then subsequently allocates more clusters, referred to herein as the
`
`15
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`auxiliary clusters, to the subscriber to increase the communication
`bandwidth. Hig-.‘:;her priorities can be given to the assignment of basic
`clusters and longer priorities 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
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`.
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`b
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`g
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`Page 21
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`1 9
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`clusters from subscribers. Alternatively, the base station may assign
`
`auxiliary clusters to one or more subscribers before allocating basic clusters
`to other subscribers. 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.
`
`10
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`From ‘to time, processing logic performs retraining 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
`
`15
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`the subscriber about the new cluster allocation.
`reselection and
`Retraining can be initiated by the base station, and in which case, the base
`
`station requestsfa specific subscriber to report its updated cluster selection.
`
`l Retraining can also be initiated by the subscriber when it observes channel
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`deterioration.
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`20'
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`Adaptive Modulation and Coding
`
`In one enébodirnent, different modulation and coding rates 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 Tablel.
`
`M
`
`P
`t
`
`'
`
`Table 1
`
`,
`
`'
`
`
`
`
`T QPSK, 1/8 Sreadin
`1
`
`1
`
`10
`
`In the example above, 1/8 spreading indicates that one QPSK
`
`modulation symbol is repeated over eight subcarriers. The
`
`repetition/spreading may also be extended to the time domain. ‘For
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`' ekample, one QPSK symbol can be repeated over four subearriers of two
`
`OFDM symbols. resulting also 1 /8 spreading.
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`Page 23
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`21
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`The cod'mg/modulation rate can be adaptively changed according to
`
`the channel conditions observed at the receiver after the initial cluster
`
`allocation and rate selection.
`
`Pilot Symbols and SINRAMeasurement
`
`10
`
`In one embodiment, each base station transmits pilot symbols
`
`simultaneously, and each pilot symbol occupies the entire OFDM frequency
`bandwidth, as shown in Figures 2A-C. Referring to Figure ZA-C, pilot
`symbols 201 are ‘shown traversing the entire OFDM frequency bandwidth
`for cells A, B
`C, respectively. In one embodiment, each of the pilot
`symbols have a length or duration of 128 microseconds with a guard time,
`the combinationgof which is approximately 152 microseconds. After each
`pilot» period, there are a predetermined number of data periods followed by
`another set of pilot symbols. In one embodiment, there are four data periods
`
`15
`
`used to transmitdata after each pilot, and each of the data periods is 152
`
`. microseconds.
`
`A subscriber estimates the SINR for each cluster from the pilot
`
`symbols. In oneiembodiment, the subscriber first estimates the channel
`
`_ response, including the amplitude and phase, as if there is no interference or
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`Page 24
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`22
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`noise. Once the channel is estimated, the subscriber calculates the
`
`interference/ noise from the received signal.
`
`The estimated SINR values may be ordered from largest to smallest
`
`SINRs and the clusters with large SINR values are selected. In one
`embodiment,
`selected clusters have SINR values that are larger than the
`
`minimum SINR which still allows a reliable (albeit low-rate) transmission
`
`supported by the system. The number of clusters selected may depend on
`
`the feedback bandwidth and the request transmission rate. In one
`
`embodiment, the subscriber always tries to send the information about as
`
`10'
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`many clusters as possible from which the base station chooses.
`
`The esdvriated SINR values are also used to choose the appropriate
`coding/modulafition rate for each cluster as discussed above. By using an
`appropriate
`indexing scheme, an SINR index may also indicate a
`
`particular coding and modulation rate that a subscriber desires to use. Note
`
`15
`
`' that even for the same subscribers, different clusters can have different
`
`modulation/coding rates.
`
`Pilot syrrlbols serve an additional purpose in determining interference
`
`among the cells. Since the pilots of multiple cells are broadcast at the same
`
`time, they will interfere with each other (because they occupy the entire
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`Page 25
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`23
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`frequency band:.' This collision of pilot symbols may be used to determine
`
`the amount of interference as a worst case scenario. Therefore, in one
`.~,;
`
`embodiment, the above SINR estimation using this method is conservative
`
`in that the measured interference level is the worst—case scenario, assuming
`
`5
`
`that all the interference sources are on. Thus, the structure of pilot symbols is
`
`such that it occupies the entire frequency band and causes collisions among
`
`different cells for use in detecting the worst case SINR in packet
`
`transmission systems.
`
`During data traffic periods, the subscribers can determine the level of
`
`i 10 , interference again. The data traffic periods are used to estimate the intra—ce1l
`
`traffic as well as: the inter-cell interference level. Specifically, the power
`
`difference during the pilot and traffic periods may be used to sense the
`
`(intra—ce1l) traffic loading and inter—cell interference to select the desirable
`
`clusters.
`
`15
`
`The interference level on certain clusters may be lower, because these
`
`clusters may befzunused in the neighboring cells. For example, in cell A, with
`
`respect to cluster A there is less interference because cluster A is unused in
`cell B (while it
`used in cell C). Similarly, in cell A, cluster B will experience
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`Page 26
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`24
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`lower mterfererfce from cell B because cluster B is used in cell B but not in
`cell C.
`3
`
`The modulation/ coding rate based on this estimation is robust to
`frequent interference changes resulted from bursty packet transmission.
`This is because
`rate prediction is based on the worst case situation in
`which all interference sources are transmitting.
`
`In one ernbodirnent, a subscriber utilizes the information available
`
`from both the pilot symbol periods and the data traffic periods to analyze
`
`the presence of both the intra-cell traffic load and inter-cell interference. The
`
`10
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`goal of the subscriber is to provide an indication to the base station as to
`
`those clusters that the subscriber desires to use. Ideally, the result of the
`
`selection by the subscriber is clusters with high channel gain, low
`interference from other cells, and high availability. The subscriber provides
`
`feedback information that includes the results, listing desired clusters in
`
`is
`
`order or not as described herein.
`
`_Figure 3 illustrates one embodiment of subscriber processing. The
`
`processing is peirforrned by processing logic that may comprise hardware
`
`(e.g., dedicated logic, circuitry, etc.), software (such as that which runs on,
`
`i-4.-d
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`25
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`for example, a