(19)
`
`(12)
`
`3 Europaisches Patentamt
`I
`European Patent Office
`
`Office européen des brevets
`
`1lllllllllllllllllllllllllllllllllllillllllllllllllllllllllllllllllllllllll
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`(11)
`
`EP 1 043 861 A1
`
`EUROPEAN PATENT APPLICATION
`
`(43) Date ot publication:
`11.10.2000 Bulletin 2000/41
`
`(21) Application number: 003015713
`
`(22) Date of filing: 28.02.2000
`
`(51) int. CL7: H04L 5/02
`
`(84) Designated Contracting States:
`AT BE CH CY DE DK ES Fl FR GB GR IE IT Ll LU
`MC NL PT SE
`Designated Extension States:
`AL LT LV MK RO SI
`
`(30) Priority: 11.03.1999 us 266371
`
`(71) Applicant:
`LUCENT TECHNOLOGIES INC.
`
`Murray Hill, New Jersey 07974-0636 (US)
`
`(72) Inventors:
`- Laroia, Rajlv
`Princeton Junction, New Jersey 08550 (US)
`- Li, Junyi
`Lakewood, New Jersey 08701 (US)
`- Vanderveen, Michaela Catalina
`Lincroft, New Jersey 07738 (US)
`
`(74) Representative:
`Buckley, Christopher Simon Thirsk
`Lucent Technologies (UK) Ltd,
`5 Mornington Road
`Woodford Green, Essex lG-8 BTU (GB)
`
`
`
`(54)
`
`Frequency hopping multicarrier transmission in segmented cells
`
`
`
`A base station within a cell of an orthogonal fre-
`(57)
`quency division multiplexing (OFDM) based spread
`spectrum multiple access system employs sectorization
`as a way to reduce the intercell interference. The cell is
`sectorized from a transmission point of view by the
`directionality of the downlink antenna, and the OFDM
`tone set employed in each cell is correspondingly sec-
`torized, i.e., each sector in the cell is allocated a set of
`tones within a sub—band of the available frequency
`bandwidth for use when transmitting into that sector.
`The sub-bands assigned to each sector are periodically
`changed, or "hopped", among the available sub-bands
`within the totally available bandwidth. Such sub—band
`hopping is a so—called "slow" hopping, in that it is not
`perlormed on a symbol—by-symbol basis but instead
`occurs only after more than one symbol has been trans-
`mitted in a sector on tones within the sub-band. Each
`
`sector errpioys its own pilot signal, which is assigned
`one or more tones within the sub-band currently
`employed by that sector. Similarly, for the uplink, the
`base station may employ a directional receiver antenna,
`Preferably, the mobile terminal only transmits on atone
`that is within a sub-band that is allocated to the sector in
`which the mobile terminal is located. This, sub-band.
`however, need not correspond to the same location
`within the bandwidth as the sub-band used by the
`downlink to communicate with the mobile terminal.
`
`EP1043861A1
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`Printed by Xe rox {UK} Business Se rvices
`2.36,?’ (HHS)/3.6
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`SPRINT 1113
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`PART 2 OF 3
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`SPRINT 1113
`PART 2 OF 3
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`EP1 043 861 A1
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`Description
`
`Technical Field
`
`This invention relates orthogonal frequency
`[0001}
`division multiplexing (OFDM) based spread spectrum
`multiple access such as may be used in wireless, and
`other, communication systems.
`
`Background of the invention
`
`is desired that wireless communication
`it
`{D0021
`systems be as efficient as possible to maximize a) the
`number of users that can be served and b) the data
`rates, if data service is provided. Wireless systems are
`shared media systems,
`i.e., there is a fixed available
`bandwidth that must be shared among all the users of
`the system. These systems are often implemented as
`so—called "cellular" systems, where the covered territory
`is divided into separate cells, and each cell is served by
`a base station.
`
`It is well known in the art that the two partic-
`[0003]
`ularly desirable features of a cellular wireless system
`are 1) that the intracell interference,
`i.e., interference
`experienced by one user that is caused by other users
`that are within the same cell as that user, be as small as
`possible, and 2) that the intercell interference. i.e., inter-
`ference experienced by one user that is caused by other
`users that are in cells other than the one in which the
`user is located, is avenged across all users in neighbor-
`ing cells. Most prior art digital cellular systems are time
`division multiple access (TDMA) systems, such as
`group special mobile (GSM)-,
`intermediate standard
`(lS)—136-, or
`lS—54—based systems, or they are code
`division multiple access (CDMA) systems, e.g.,
`lS—95
`based systems.
`[0004]
`In prior art narrow band TDMA systems
`neighboring base stations use different, e.g., non—over-
`lapping, parts of the available spectrum. However,
`bases stations that are sufficiently far away from each
`other to avoid substantial interference between them,
`i.e., normeighboring base stations, may use the same
`parts of the available spectrum. Notwithstanding such
`spectrum reuse, the spectrum available for use in each
`cell is a small part of the total available spectrum. Each-
`user in a cell has its own unique frequency band and
`time slot combination, and hence TDMA systems have
`no intracell interference, i.e., they have the first desira-
`ble feature of cellular wireless systems. However,
`TDMA systems do not have the second desirable fea-
`ture,
`in that a given user only interferes with a small
`number of users outside the cell, so that spectral reuse
`is based on worst case interference rather than average
`interference. As a result, the system has a low "spectral"
`efficiency
`in prior art direct sequence (DS)-CDMA sys-
`[0005]
`tems the entire bandwidth is used by each base station
`but each base station uses a different spreading code.
`
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`Such CDMA systems promise higher spectral efficiency
`than narrow band TDMA systems. Thus, CDMA sys-
`tems have the second desirable feature of a cellular
`wireless system. However, CDMA systems do not have
`the first desirable feature of a cellular wireless system
`because although the signals transmitted from the base
`station within a cell are orthogonal, because of channel
`dispersion, the signals received at a receiver are not
`necessarily orthogonal. This results in interference
`between users within the same cell.
`
`Proposed prior art frequency hopping (FH)-
`[0006]
`CDMA systems are very similar to narrow band TDMA
`systems, except that they employ frequency hopping to
`also obtain the second desirable feature of a cellular
`wireless system.
`in particular, each transmitter trans-
`mits a narrow band signal, and periodically changes the
`carrier frequency to achieve the frequency hopping.
`However, disadvantageously, such hopping is relatively
`slow, reducing the amount of averaging that can be
`achieved for a given delay in the transmission path that
`the system can tolerate.
`[0007]
`United States Patent No. 5,410,538 issued
`to Floche et al. on April 25, 1995 discloses a multi-tone
`CDMA system. Such a system is essentially an OFDM
`system that eliminates intracell interference by insuring
`that the received signals within a cell are orthogonal.
`Thus, the Roche et al. system has both desirable fea-
`tures of a cellular wireless system. However, the Roche
`et al. system partitions the spectrum into a large
`number of tones, which makes the system very suscep-
`tible to Doppler shifts in mobile systems. Also, because
`each mobile user transmits on a large number of tones.
`the peak-to—average ratio of the mobile transmitter is
`very high, resulting in poor power efficiency at the
`mobile station, which is disadvantageous in that power
`is often a limited resource in the mobile station.
`[0008]
`United States Patent No. 5,548,582 issued
`to Brajal et al. on August 20, 1996 discloses a general
`wide-band orthogonal frequency division multiplexing
`(OFDM) based spread spectrum multiple access.
`[0009]
`We have recognized in United States Patent
`Application Seriai No. (Case Laroia 9-1-1) that the Bra-
`jal et at. system is not optimized for use in a cellular sys-
`tem in that there is no teaching therein how to optimize
`a) the hopping pattern, b) the tone assignment, or c) the
`bandwidth reuse. We have further recognized that opti-
`mizing these factors, individually and/or collectively,
`is
`important to obtain a spectrally efficient system, i.e., a
`system that has the two particularly desirable features
`of a cellular wireless system. in particular, we disclosed
`in united States Patent Application Serial No. (Case
`Laroia 9-1-1) dividing the entire bandwidth into orthogo-
`nal tones, and reusing all of the orthogonal tones in
`each cell. To reduce peak—to-average ratio at the mobile
`transmitter, low bit rate user, such as a voice user,
`is
`allocated preferably a single one, but no more than a
`very small number, of the orthogonal tones for use in
`communicating with the base station. Data users are
`
`Page 243
`
`

`
`3
`
`EP1043 861 A1
`
`4
`
`similarly allocated tones for data communication. How-
`ever, the number of tones assigned for each data partic-
`ular user is a function of the data rate for that user. The
`tone assignment for a given user is not always the same
`within the available band, but
`instead the tones
`assigned to each user are hopped over time.
`[0010]
`A tone hopping pattern was disclosed that
`achieves maximum frequency diversity and averages
`the intercell interference, e.g., using a pattern that is a
`function of a mutually orthogonal latin square. More
`specitically, in the downlink, ie, in the channel from the
`base station to the mobile station, the tones assigned to
`each user are change relatively rapidly, e.g., from sym-
`bol to symbol, i.e., the user fast "hops" from one tone to
`another. However, in the uplink, ie, in the channel from
`the mobile station to the base station, although fast hop-
`ping is possible, preferably slow hopping is employed to
`allow efficient modulation of the uplink signal. However,
`when slow hopping is used in the uplink, it is necessary
`to empioy additional techniques, such as interleaving, to
`compensate for the reduction in the intercell interfer-
`ence averaging effect.
`
`Summary of the invention
`
`We have recognized that notwithstanding
`[0011]
`the foregoing advancements, additional improvements
`are yet necessary to achieve a spectrally efficient sys-
`tem, i.e., a system that has the two particularly desirable
`features of a cellular wireless system. One such
`improvement, in accordance with the principles of the
`invention, is the use by a base station within a cell of a
`directional antenna in order to be able to employ sector-
`ization as a way to reduce the intercell interference. in
`accordance with an aspect of the invention, not only is
`the cell sectorized from a transmission point of view by
`the directionality of
`the downlink antenna, but the
`OFDM tone set employed in each cell is correspond-
`ingly sectorized, i.e., each sector in the cell is allocated
`a set of tones within a sub—band of the available fre-
`
`quency bandwidth for use when transmitting into that
`sector. For example, with hexagonally shaped calls, all
`the sectors with the same directional orientation are
`allocated tones within the same sub-band.
`
`in accordance with another aspect of the
`[0012]
`invention, the sub-bands assigned to each sector are
`periodically changed, or "hopped", among the available
`sub-bands within the totally available bandwidth. Such
`sub—band hopping is a so-called "slow" hopping, in that
`it is not performed on a symbol-by-symbol basis but
`instead occurs only after more than one symbol has
`been transmitted in a sector on tones within the sub-
`band. Furthermore, the slow hopping of the sub-bands
`can be no faster than slow hopping which may be
`employed in the uplink. i.e., the link from the mobile ter-
`minal to the base station, such as is described in United
`States Patent Application Serial No. (Case Laroia EH-
`1).
`in the downlink, each sector employs its own pilot
`
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`signal, which is assigned one or more ones within the
`sub-band currently employed by that sector.
`[0013]
`Similarly, for the uplink, the base station may
`employ a directional receiver antenna. Preferably,
`in
`accordance with an aspect of the invention, the mobile
`terminal only transmits on a tone that is within a sub-
`band that is allocated to the sector in which the mobile
`terminal is located. This, sub—band, however, need not
`correspond to the same location within the bandwidth
`as the sub—band used by the downlink to communicate
`with the mobile terminal.
`
`Brief Description of the Drawing
`
`[0014]
`
`In the drawing:
`
`Fla 1 shows an example of available orthogonal
`tones at one cell with a spacing of it within a band-
`width W;
`FIG. 2 shows a time domain view of the symbol
`period which is available for symbol transmission,
`and the additional time required for transmission of
`the cyclic prefix;
`FlG. 3 shows a block diagram of an exemplary
`OFDM transmitter;
`Fla 4 shows a block diagram of an exemplary
`OFDM receiver;
`FIG. 5 shows further details 01 an exemplary imple-
`mentation of data-to—tone applier of FlG. 3 for a
`base station;
`FlG. 6 shows several contiguous hexagonally
`shaped cells;
`HQ. 7 shows one allocation of sub—band assign-
`ment for sets of the hexagonally shaped sectors of
`HG. 6;
`sub—band
`FIG. 8 shows another allocation of
`assignment for sets of the hexagonally shaped sec-
`tors of FIG. 6; and
`HG. 9 shows an exemplary hopping pattern from
`one user, in accordance with the principles of the
`invention.
`
`Detailed Description
`
`The following merely illustrates the princi-
`{(3015}
`ples of the invention.
`it will thus be appreciated that
`those skilled in the art will be able to devise various
`arrangements which, although not explicitly described
`or shown herein, embody the principles of the invention
`and are included within its spirit and scope. Further-
`more, all examples and conditional language recited
`herein are principally intended expressly to be only for
`pedagogical purposes to aid the reader in understand-
`ing the principles ot the invention and the concepts con-
`tributed by the inventor(s) to furthering the art, and are
`to be construed as being without limitation to such spe-
`cifically recited examples and conditions. Moreover, all
`statements herein reciting principles, aspects, and
`
`Page 244
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`

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`5
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`EP1 043861 A1
`
`6
`
`embodiments of the invention, as well as specific exam-
`ples thereof, are intended to encompass both structural
`and functional equivalents thereof. Additionally,
`it
`is
`intended that such equivalents include both currently
`known equivalents as well as equivalents developed in
`the future, ie. any elements developed that perform the
`same function, regardless of structure.
`[0016]
`Thus, for example, it will be appreciated by
`those skilled in the art that the block diagrams herein
`represent conceptual views of
`illustrative circuitry
`embodying the principles of the invention. Similarly,
`it
`will be appreciated that any flow charts, flow diagrams,
`state transition diagrams, pseudocode, and the like rep-
`resent various processes which may be substantially
`represented in computer readable medium and so exe-
`cuted by a computer or processor, whether or not such
`computer or processor is explicitly shown.
`[0017]
`The functions of the various elements shown
`in the F lGs.,
`including functional blocks labeled as
`"processors" may be provided through the use of dedi-
`cated hardware as well as hardware capable of execut-
`ing software in association with appropriate software.
`When provided by a processor, the functions may be
`provided by a single dedicated processor. by a single
`shared processor, or by a plurality of individual proces-
`sors, some of which may be shared. Moreover, explicit
`use of the term "processor" or "controller" should not be
`construed to refer exclusively to hardware capable of
`executing software, and may implicitly include, without
`limitation, digital signal processor (DSP) hardware,
`read-only memory (ROM) for storing software, random
`access memory (RAM), and non-volatile storage. Other
`hardware, conventional and/or custom, may also be
`included. Similarly, any switches shown in the FIGS. are
`conceptual only. Their function may be carried out
`through the operation of program logic, through dedi-
`cated logic, through the interaction of program control
`and dedicated logic, or even manually, the particular
`technique being selectable by the implementor as more
`specifically understood from the context.
`[0018]
`in the claims hereof any element expressed
`as a means for performing a specified function is
`intended to encompass any way of performing that func-
`tion including, for example, a) a combination of circuit
`elements which performs that function or b) software in
`any form,
`including, therefore, firmware. microcode or
`the like, combined with appropriate circuitry for execut-
`ing that software to perform the function. The invention
`as defined by such claims resides in the fact that the
`functionalities provided by the various recited means
`are combined and brought together in the manner which
`the claims call for. Applicant thus regards any means
`which can provide those functionalities as equivalent as
`those shown herein.
`
`Before describing the invention it is neces-
`[0019]
`sary to understand generally the environment in which
`the invention operates, namely, orthogonal frequency
`‘ division multiplexing (OFDM) systems.
`
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`Orthogonal frequency division multiplexing
`[0020]
`(OFDM) systems employ orthogonal tones within afre-
`quency bandwidth to transmit data from different users
`at the same time. in particular, for any particular symbol
`period T which is available for symbol transmission, and
`a given bandwidth W, the number of available orthogo-
`nal tones N,
`is given by WT.
`in accordance with an
`aspect of the invention,
`the same bandwidth W is
`reused in each cell. The spacing between the orthogo-
`nal tones is A, which is given by i/T.
`in addition to the
`symbol period T which is available for symbol transmis-
`sion, an additional time To is required for transmission
`of a cyclic prefix, which is prepended to each symbol
`period and is used to compensate for the dispersion
`introduced by the channel response and the pulse
`shaping filter used at the transmitter. Thus, although a
`total period of T+Tc is employed, only T is available for
`user data transmission.
`
`shows an example of available
`1
`FIG.
`[0021]
`orthogonal tones at one cell with a spacing of A within a
`bandwidth W. FlG. 2 shows a time domain view of the
`
`symbol period T which is available for symbol transmis-
`sion, and the additional time To required for transmis-
`sion of the cyclic prefix, Note that within each symbol
`period T data may be sent on each of the tones sub-
`stantiaily simultaneously. Also, the last portion of the
`data symbol period T is often employed as the cyclic
`prefix in manner shown in F IG. 2.
`[0022]
`FlG. 3 shows a block diagram of exemplary
`OFDM transmitter 301. Because of
`its high level,
`whether or not the diagram of FlG. 3 depicts a prior art
`OFDM transmitter or an OFDM in accordance with the
`
`principles of the invention depends on the particular
`implementation of the various components of H6. 3.
`Also, OFDM transmitter 301 may be used in either a
`base station as the downlink transmitter or in a mobile
`station as an uplink transmitter. The particular embodi-
`ments necessary for either application will be described
`more fully hereinbelow.
`[0023]
`OFDM transmitter 301 includes a) encoder
`303, b) data—to-tone applier 305, c) tone assignment
`unit 307, and d) cyclic prefix prepender 809.
`[0024]
`Encoder 303 receives an overall information
`stream for transmission from a source and encodes it
`according to a particular encoding scheme. Such over-
`all
`lnformation stream typically includes information
`streams generated on behalf of more than one user if
`OFDM transmitter 301 is being used in a base station
`and only includes information streams for one user if
`OFDM transmitter 301 is being used in a mobile station.
`The encoding scheme employed may vary whether the
`information being transmitted in a particular information
`stream is voice or data. Those of ordinary skill in the art
`will be able to 1) select, e.g., traditional convolutional or
`block coding, or 2) devise, appropriate encoding
`schemes as a function of the model of the interference
`environment
`in which the OFDM system is being
`deployed.
`
`Page 245
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`

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`EP 1 043 861 A1
`
`8
`
`'-"U153.
`Data-to—tc-ne applier uuv modutates the
`{@0251
`overall encoded information stream supplied as an out-
`put from encoder 303 onto the various available tones.
`For each particular encoded information stream within
`the overall encoded information stream at least one
`
`tone is assigned by tone assignment unit 307, and that
`tone is used to modulate the particular encoded infor-
`mation stream received from encoder 303. it a particu-
`lar encoded information stream is voice then,
`in
`accordance with an aspect of the invention, preferably a
`single one, but no more than a very small number, of the
`orthogonal tones are assigned for particular encoded
`information stream.
`if a particular encoded information
`stream is data then, in accordance with an aspect of the
`invention, the number of orthogonal tones assigned that
`particular encoded information stream is a function of
`the data rate for the user of that particular encoded
`information stream.
`
`Tones are assigned to each encoded infor-
`[0026]
`mation stream by tone assignment unit 307, which con-
`veys the assignments to data-to-tone applier 305. The
`tone assignment for a given user is not always the same
`within the available band, but
`instead the tones
`assigned to each user are hopped over time by tone
`assignment unit 307.
`[0027]
`Cyclic prefix prepender 309 adds the cyclic
`prefix to each symbol period as described above. The
`cyclic prefix is added only for the tones being used by
`OFDM transmitter 301. Thus, for example,
`if OFDM
`transmitter 301 is in a base station using all of the tones,
`then the cyclic prefix uses all of the available orthogonal
`tones within bandwidth W. it OFDM transmitter 301 is in
`
`a mobile station using only a single one of the tones,
`then the cyclic prefix uses only that particular single
`tone. Advantageously, use of the cyclic prefix eliminates
`the need for equalization at the receiver.
`[0028]
`FIG. 4 shows a block diagram of an exem-
`plary OFDM receiver 401. As with FlG. 3 because of its
`high level, whether or not the diagram of FIG. 4 depicts
`a prior art OFDM receiver or an OFDM in accordance
`with the principles of the invention depends on the par-
`ticular implementation of the various components of
`FIG. 4. Also, as shown OFDM receiver 401 may be used
`in either a base station as the downlink receiver or in a
`
`mobile station as an uplink receiver. The particular
`embodiments necessary for either application will be
`described more fully hereinbelow.
`[0029]
`OF DM receiver 401 includes a) cyclic prefix
`remover 409, b) tone-to-data extractor 405, c) tone
`assignment unit 407, and d) decoder 403.
`[0030]
`The signal received at OFDM receiver 401,
`e.g., by an antenna and amplifier arrangement, not
`shown, is supplied to cyclic prefix remover 409. Cyclic
`prefix remover 409 removes the cyclic prefix from each
`total period of the received signal. The remaining signal,
`with period T, is supplied to tone—todata extactor 405.
`[0031]
`Tone-to—data extractor 405 extracts each
`information stream received on the various available
`
`ones which are being used by OFDM receiver 401 to
`develop an overall reconstructed data stream. Tones
`are assigned for use by OFDM receiver 401 by tone
`assignment unit 407, which conveys the assignments to
`data-to—tone remover 405. The tone assignment for a
`given user is not always the same within the available
`band, but instead the tones assigned to each user are
`hopped over time by tone assignment unit 407. As a
`resuit,
`it
`is necessary that there be correspondence
`between tone assignment unit 307 of OFDM transmitter
`301 and tone assignment unit 407 of an associated
`OFDM receiver 401. Such correspondence is typically
`achieved through a priori arrangement, e.g., upon call
`set up.
`Decoder 403 receives an overall information
`[0032]
`stream from transmission tone-to-data extractor 405
`
`and decodes it to develop an overall output information
`stream. The decoding is often performed according to
`the inverse of the scheme used to encode the informa-
`tion stream. However, modifications may be made to the
`decoding scheme to account for channel and other
`effects to produce a more reliable decoded output than
`simply using the inverse of the encoding scheme. Alter-
`natively, specific algorithms may be developed for use in
`decoding the received signal
`that take into account
`channel response, interference, and other effects. Such
`overall output
`information stream typically includes
`information streams generated on behalf of more than
`one user if OFDM receiver 401 is being used in a base
`station and only includes information streams for one
`user if OFDM receiver 401 is being used in a mobile sla-
`tion.
`
`The resulting overall output stream is sup-
`[0033]
`plied to a destination tor further pocessing. For exam-
`ple,
`if
`the information stream is voice and OFDM
`receiver 401 is within a mobile station, then the informa-
`tion stream is supplied to be converted to an audible
`signal that is played for the user.
`if the information
`stream is voice and OFDM receiver 401 is within a base
`station, the voice information may be separated for
`transmission to the ultimate destination, e.g., via a wire-
`line network.
`
`FlG. 5 shows further details of an exemplary
`[0034]
`implementation of data-to-tone applier 305 for a base
`station. Each of multipliers 501 multiplies a particular
`intormation stream by a sinusoidal waveform which is
`one of the orthogonal tones and is generated by tone
`generator 503. The resulting modulated signals are
`then summed by adder 505. Typically, data—to-tone
`applier 305 is implemented digitally, e.g., by a processor
`performing the functionality of multipliers 501, tone gen-
`erator 503, and adder 505 using digital representations
`of the orthogonal tones.
`{0035}
`The same general architecture as shown in
`FIG. 5 may be used to implement data-to-tone applier
`305 for a mobile station. However, instead of covering
`the entire range of N orthogonai tones used within the
`cell by the base station by having N multipliers, only the
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`maximum number of orthogonal tones used by the
`mobile station need have available multipliers. Since
`many mobile stations are used strictly for voice, only
`one multiplier need be provided. However, since, as will
`be described in more detail hereinbelow,
`the tone
`assignments for each user are changed, it is necessary
`that the tone generator in a mobile station be able to
`generate the entire range of N orthogonal tones. Fur-
`thermore, if only one tone is used, adder 505 may be
`dispensed with.
`[0036]
`As described above, the tones assigned to
`any particular information stream is changed periodi-
`cally. This is known in the art generally as frequency
`hopping. and is referred to herein more specifically as
`tone hopping.
`the
`[0037]
`in accordance with the principles of
`invention,
`in OFDM systems, the antenna ultimately
`transmitting the overall encoded information stream as
`applied to the various available tones may be a direc-
`tional antenna so that sectorization of the cell may be
`employed as a way to reduce the intercell interference.
`in accordance with an aspect of the invention, not only
`is the cell sectcrized from a transmission point of view
`by the directionality of the downlink antenna, but the
`OFDM tone set employed in each cell is correspond-
`ingly sectorized, i.e., each sector in the cell is allocated
`a set of tones within a sub—band oi the available fre-
`quency bandwidth for use when transmitting into that
`sector.
`
`FlG. 6 shows several contiguous hexago-
`[0038]
`nally shaped cells 601. Within hexagonally shaped cells
`601 , all sectors with the same directional orientation,
`e.g., sets of sectors 603, 605 and 607, are allocated
`tones within the same sub—band, as indicated by labels
`1, 2, and 3. FIG. 7 shows one allocation of sub—band
`assignment for sets of sectors 603, 605, and 607 to sub-
`bands 1, 2, and 3, respectively.
`in accordance with
`another aspect
`of
`the invention,
`the sub-bands
`assigned to each sector are periodically changed, or
`"hopped", among the available sub-bands within the
`totally available bandwidth. Thus, FIG. 8 shows another,
`e.g., later, allocation of sub-band assignment for sets of
`sectors 603, 605, and 607 to sub—bands 3, 1, and 2,
`respectively.
`[0039]
`Such .sub-band hopping is preferably a so—
`called “slow" hopping, in that it is not performed on a
`symbol-by-symbol basis but instead occurs only after
`more than one symbol has been transmitted in a sector
`on tones within the sub—band. Furthermore, the slow
`hopping of the sub-bands can be no faster than the slow
`hopping that may be employed in the uplink, i.e., the link
`from the mobile terminal to the base station, such as is
`described in United States Patent Application Serial No.
`(Case Laroia 9-1-1).
`{D040}
`H62. 9 shows an exemplary hopping pattern
`for the mobile terminal of one user, in accordance with
`the principles of the invention. Each column represents
`a symbol or set of symbols that is transmitted, for a
`
`given time period. The length of the time period and the
`length of the symbol period T determine whether each
`column represents a single symbol or a set of symbols.
`However, preterabl y, in the downlink each column repre-
`sents a single symbol and in the uplink each column
`represents a set of symbols. The rows of H6. 9 repre-
`sents atone that is used to transmit the user's symbol or
`set of symbols. The tones included within sub-bands 1,
`2, and 3. are identified on the Y-axis of HG. 9.
`
`According to the exemplary hopping pattern
`{OD411
`of FIG. 9, the user's mobile terminal first experiences
`several symbol periods, e.g., 5, within sub-band 1 using
`various tones therein, at which point the sector the
`user's mobile terminal
`is located in switches to sub-
`band 2. The user's mobile terminal then experiences
`several symbol periods within sub-band 2, at which
`point the sector the user's mobile terminal is located in
`switches to sub-band 3. After 3 syrrbol periods,
`the
`user's mobile terminal leaves the sector it was located in
`and enters another sector of the same cell that is using
`tones in sub—band 2. The user‘s mobile terminal thus
`
`experiences an additional two symbol periods in sub-
`band 2, at which point the new sector in which the users
`mobile terminal became located in hops to sub-band 3.
`The user's mobile terminal then experiences 5 symbols
`in sub-band 3. Finally, the new sector that the user’s
`mobile terminal is located in hops back to sub—band t,
`and the user's mobile terminal employs 5 symbols in
`sub-band 1, whereupon the user turns off his mobile ter-
`minal.
`
`Note that there is no requirement for user to
`{O04-2]
`experience each tone in a sub—band before the sub-
`band employed by the user is changed.
`[0043]
`The sub—bands assigned to the various sec-
`tors are preferably,
`identical in bandwidth, as well as
`contiguous, continuous, and nonoverlapping in the fre-
`quency domain. Note, however that the sub-bands
`assigned to the various sectors may be overlapping as
`such an arrangement may be able to achieve higher
`capacity for the entire system. Additionally,
`the sub-
`bands need not have identical bandwidths, so that some
`sub~bands may include more tones than other sub-
`bands. Furthermore, the tones making up a sub—band
`may change dynamically. in fact, the tones making up a
`sub—band need not be contiguous in the frequency
`domain. However,
`the system with contiguous sub
`bands may require less tones for use as pilot signals.
`[0044]
`A pilot signal is a signal that is known to the
`receiver, and so the pilot signal as received can be used
`for purposes such as channel estimation, eg., by figur-
`ing out the operation performed by the channel to the
`pilot signal as transmitted in order to develop the pilot
`signal as received. in accordance with an aspect of the
`invention, each sector employs its own pilot signal,
`which is assigned one or more tones within the sub-
`band currently employed by that sector. Thus, for the
`above example shown in FlGs. 6-8, there would be
`three pilot signals in the downlink, one within sub-band
`
`10
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`15
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`1, another within sub-band 2, and a third within sub-
`band 3. The tones used for the pilot signal are hopped
`along with all the other tones carrying user information.
`It more than one tone is employed as the pilot signal, the
`tones making up the pilot signal may be separated from
`each other. Such separation may be used to achieve
`better channel estimation, because, by distributing the
`tones used by the pilot signal across the sub-bands. the
`channel effects experienced by the tones of the pilot sig-
`nal are more likely to be repres