(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2002/0055356 A1
`Dulin et al.
`(43) Pub. Date:
`May 9, 2002
`
`US 20020055356A1
`
`(54) SYSTEM AND METHOD FOR
`SYNCHRONIZING DATA TRANSMISSION
`FROM MULTIPLE WIRELESS BASE
`TRANSCEIVER STATIONS TO A
`SUBSCRIBER UNIT
`
`(76) Inventors: David R. Dulin, San Francisco, CA
`(US); Sanjay Kasturia, Palo Alto, CA
`(US); Partho Mishra, Cupertino, CA
`(US); Arogyaswami J. Paulraj,
`Stanford, CA (US); Matthew S. Peters,
`Mountain View, CA (US)
`
`Correspondence Address:
`Patent Department
`Iospan Wireless
`P.O. Box 641867
`San Jose, CA 95164-1867 (US)
`
`(21) Appl. No.:
`
`09/729,886
`
`(22) Filed:
`
`Dec. 4, 2000
`
`Related US. Application Data
`
`(63) Continuation-in-part of application No. 09/708,170,
`?led on Nov. 7, 2000.
`
`Publication Classi?cation
`
`(51) Im. c1? ..................................................... .. H04Q 7/20
`
`(52) US. Cl. ....................... .. 455/422; 455/452; 455/456;
`455/502
`
`(57)
`
`ABSTRACT
`
`The invention includes an apparatus and a method for
`transmitting sub-protocol data units from a plurality of base
`transceiver stations to a subscriber unit. The method
`includes estimating time delays required for transferring the
`sub-protocol data units betWeen a scheduler unit and each of
`the base transceiver stations. The method further includes
`the scheduler unit generating a schedule of time slots and
`frequency blocks in Which the sub-protocol data units are to
`be transmitted from the base transceiver stations to the
`subscriber unit. The time delays are used to generate the
`schedule. The time delays can be used to generate a look
`ahead schedule that compensates for the timing delays of the
`sub-protocol data units from the scheduler unit to the base
`transceiver stations. The sub-protocol data units are Wire
`lessly transmitted from the base transceiver stations to the
`subscriber unit according to the schedule. The time delays
`can be estimated by time-stamping sub-protocol data units
`before sub-protocol data units are transferred from the
`scheduler unit to the base transceiver stations, and estimat
`ing the time delays by comparing the times the sub-protocol
`data units are actually received by the base transceiver
`stations With the time-stamping.
`
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`ERIC-1002
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`Page 1 of 32
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`Patent Application Publication May 9, 2002 Sheet 1 0f 20
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`Patent Application Publication May 9, 2002 Sheet 6 0f 20
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`Patent Application Publication May 9, 2002 Sheet 7 0f 20
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`ERIC-1002
`Page 8 of 32
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`Patent Application Publication
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`May 9, 2002 Sheet 9 0f 20
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`Page 10 of 32
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`Patent Application Publication May 9, 2002 Sheet 10 of 20 US 2002/0055356 A1
`
`Receive Standard Protocol Data Units from Network
`
`Sub-divide Standard Protocol Data Units into Sub-Protocol Data Units
`
`Buffer the Sub-Protocol Data Units
`
`Estimate Time Delays Required for Transferring Sub-Protocol Data Units to
`Base Transceiver Stations
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`
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`ERIC-1002
`Page 11 of 32
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`Patent Application Publication May 9, 2002 Sheet 11 of 20 US 2002/0055356 A1
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`ERIC-1002
`Page 12 of 32
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`

`
`Patent Application Publication May 9, 2002 Sheet 12 0f 20 US 2002/0055356 A1
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`ERIC-1002
`Page 13 of 32
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`Patent Application Publication May 9, 2002 Sheet 13 0f 20 US 2002/0055356 A1
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`ERIC-1002
`Page 14 of 32
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`

`
`Patent Application Publication May 9, 2002 Sheet 14 0f 20 US 2002/0055356 A1
`
`
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`ERIC-1002
`Page 15 of 32
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`Patent Application Publication May 9, 2002 Sheet 15 0f 20 US 2002/0055356 A1
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`Patent Application Publication
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`May 9, 2002 Sheet 18 of 20 US 2002/0055356 A1
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`ERIC-1002
`Page 19 of 32
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`Patent Application Publication May 9, 2002 Sheet 19 0f 20 US 2002/0055356 A1
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`ERIC-1002
`Page 20 of 32
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`Patent Application Publication May 9, 2002 Sheet 20 of 20
`
`US 2002/0055356 A1
`
`Fig. 17
`
`ERIC-1002
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`Page 21 of 32
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`ERIC-1002
`Page 21 of 32
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`

`
`US 2002/0055356 A1
`
`May 9, 2002
`
`SYSTEM AND METHOD FOR SYNCHRONIZING
`DATA TRANSMISSION FROM MULTIPLE
`WIRELESS BASE TRANSCEIVER STATIONS TO A
`SUBSCRIBER UNIT
`
`CROSS-REFERENCE TO RELATED
`APPLICATION
`
`[0001] This application is a Continuation-in-Part of U.S.
`patent application Ser. No. 09/708,170, filed Nov. 7, 2000.
`
`FIELD OF THE INVENTION
`
`[0002] The invention relates generally to wireless com-
`munications. More particularly,
`the invention relates to
`synchronizing transmission of data between multiple base
`transceiver stations and subscriber units, providing spatial
`multiplexing and communication diversity.
`
`BACKGROUND OF THE INVENTION
`
`commonly
`systems
`communication
`[0003] Wireless
`include information carrying modulated carrier signals that
`are wirelessly transmitted from a transmission source (for
`example, a base transceiver station) to one or more receivers
`(for example, subscriber units) within an area or region.
`
`[0004] Spatial Multiplexing
`
`[0005] Spatial multiplexing is a transmission technology
`that exploits multiple antennae at both the base transceiver
`station and at the subscriber units to increase the bit rate in
`
`a wireless radio link with no additional power or bandwidth
`consumption. Under certain conditions, spatial multiplexing
`offers a linear increase in spectrum efficiency with the
`number of antennae. For example, if three antennae are used
`at the transmitter (base transceiver station) and the receiver
`(subscriber unit), the stream of possibly coded information
`symbols is split into three independent substreams. These
`substreams occupy the same channel of a multiple access
`protocol. Possible same channel multiple access protocols
`include a same time slot in a time-division multiple access
`protocol, a same frequency slot in frequency-division mul-
`tiple access protocol, a same code sequence in code-division
`multiple access protocol or a same spatial target location in
`space-division multiple access protocol. The substreams are
`applied separately to the transmit antennae and transmitted
`through a radio channel. Due to the presence of various
`scattering objects in the environment, each signal experi-
`ences multipath propagation.
`
`[0006] The composite signals resulting from the transmis-
`sion are finally captured by an array of receiving antennae
`with random phase and amplitudes. At the receiver array, a
`spatial signature of each of the received signals is estimated.
`Based on the spatial signatures, a signal processing tech-
`nique is applied to separate the signals, recovering the
`original substreams.
`
`[0007] FIG. 1 shows three transmitter antenna arrays 110,
`120, 130 that transmit data symbols to a receiver antenna
`array 140. Each transmitter antenna array includes spatially
`separate antennae. A receiver connected to the receiver
`antenna array 140 separates the received signals.
`
`[0008] FIG. 2 shows modulated carrier signals traveling
`from a transmitter 210 to a receiver 220 following many
`different (multiple) transmission paths.
`
`[0009] Multipath can include a composition of a primary
`signal plus duplicate or echoed images caused by reflections
`of signals off objects between the transmitter and receiver.
`The receiver may receive the primary signal sent by the
`transmitter, but also receives secondary signals that are
`reflected off objects located in the signal path. The reflected
`signals arrive at the receiver later than the primary signal.
`Due to this misalignment, the multipath signals can cause
`intersymbol interference or distortion of the received signal.
`
`[0010] The actual received signal can include a combina-
`tion of a primary and several reflected signals. Because the
`distance traveled by the original signal is shorter than the
`reflected signals, the signals are received at different times.
`The time difference between the first received and the last
`
`received signal is called the delay spread and can be as great
`as several micro-seconds.
`
`[0011] The multiple paths traveled by the modulated car-
`rier signal typically results in fading of the modulated carrier
`signal. Fading causes the modulated carrier signal to attenu-
`ate in amplitude when multiple paths subtractively combine.
`
`[0012] Communication Diversity
`
`[0013] Antenna diversity is a technique used in multiple
`antenna-based communication system to reduce the effects
`of multi-path fading. Antenna diversity can be obtained by
`providing a transmitter and/or a receiver with two or more
`antennae. These multiple antennae imply multiple channels
`that suffer from fading in a statistically independent manner.
`Therefore, when one channel is fading due to the destructive
`effects of multipath interference, another of the channels is
`unlikely to be suffering from fading simultaneously. By
`virtue of the redundancy provided by these independent
`channels, a receiver can often reduce the detrimental effects
`of fading.
`
`transmission link exists between
`[0014] An individual
`each individual base transceiver station antenna and a sub-
`scriber unit in communication with the base transceiver
`
`station. The previously described spatial multiplexing and
`communication diversity require multiple antennas to each
`have transmission links with a single subscriber unit. Opti-
`mally, the base transceiver station can schedule data trans-
`mission according to the transmission link quality.
`
`It is desirable to have an apparatus and method that
`[0015]
`provides scheduling of transmission of data blocks between
`multiple base station transceivers and receivers (subscriber)
`units. It is desirable that the scheduling be adaptive to the
`quality of transmission links between the base station trans-
`ceivers and the receivers (subscriber) units. It is additionally
`desirable that the apparatus and method allow for spatial
`multiplexing and communication diversity through the mul-
`tiple base station transceivers.
`
`SUMMARY OF THE INVENTION
`
`[0016] As shown in the drawings for purposes of illustra-
`tion, the invention is embodied in an apparatus and a method
`for scheduling wireless transmission of data blocks between
`multiple base transceiver stations and multiple receiver
`(subscriber) units. The scheduling accounts for time delays
`that exist between a scheduler unit and the base transceiver
`
`stations. The scheduling can be based on the quality of a
`transmission link between the base transceiver stations and
`
`the receiver units,
`
`the amount of data requested by the
`
`ERIC-1002
`
`Page 22 of 32
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`Page 22 of 32
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`

`
`US 2002/0055356 A1
`
`May 9, 2002
`
`receiver units, and/or the type of data requested by the
`receiver units. The scheduling generally includes assigning
`frequency blocks and time slots to each of the receiver units
`for receiving or transmitting data blocks. The transmission
`scheduling allows for spatial multiplexing and communica-
`tion diversity through spatially separate base station trans-
`ceivers.
`
`[0017] A first embodiment of the invention includes a
`method of transmitting sub-protocol data units from a plu-
`rality of base transceiver stations to a subscriber unit. The
`method includes estimating time delays required for trans-
`ferring the sub-protocol data units between a scheduler unit
`and each of the base transceiver stations. The method further
`
`includes the scheduler unit generating a schedule of time
`slots and frequency blocks in which the sub-protocol data
`units are to be transmitted from the base transceiver stations
`to the subscriber unit. This embodiment can include the time
`
`delays being used to generate the schedule.
`
`[0018] A second embodiment of the invention is similar to
`the first embodiment. The second embodiment
`further
`
`includes the time delays being used to generate the schedule
`by using the time delays to project the timing of when the
`sub-protocol data units are to be wirelessly transmitted from
`the base transceiver stations.
`
`to the second
`is similar
`[0019] A third embodiment
`embodiment. The third embodiment
`includes a the time
`
`delays being used to generate a look ahead schedule that
`compensates for the timing delays of transferring the sub-
`protocol data units from the scheduler unit
`to the base
`transceiver stations.
`
`[0020] A fourth embodiment is similar to the first embodi-
`ment. The fourth embodiment includes wirelessly transmit-
`ting the sub-protocol data units from the base transceiver
`stations to the subscriber unit according to the schedule.
`
`[0021] A fifth embodiment is similar to the first embodi-
`ment. The fifth embodiment includes the estimating time
`delays required for transferring the sub-protocol data units
`between the scheduler unit and the base transceiver stations
`
`by time-stamping sub-protocol data units before sub-proto-
`col data units are transferred from the scheduler unit to the
`
`base transceiver stations, and estimating the time delays by
`comparing the times the sub-protocol data units are actually
`received by the base transceiver stations with the times of
`the time-stamping.
`
`[0022] A sixth embodiment is similar to the first embodi-
`ment. The sixth embodiment includes the scheduler receiv-
`
`ing standard protocol data units from a network and sub-
`dividing the standard protocol data units forming the sub-
`protocol data units.
`
`to the first
`is similar
`[0023] A seventh embodiment
`embodiment. The seventh embodiment includes synchroniz-
`ing the base transceiver stations to a common reference
`clock. The synchronization can include receiving a global
`positioning satellite (GPS) signal, and generating the com-
`mon reference clock from the GPS signal.
`
`[0024] A eighth embodiment is similar to the first embodi-
`ment. The eighth embodiment includes the sub-protocol data
`units being transmitted between the base transceiver stations
`and the subscriber unit in data blocks, the data blocks being
`defined by a frequency block and time slot. Generally, the
`
`scheduler unit generates a map that determines when the
`data blocks are transmitted the base transceiver stations and
`the subscriber unit.
`
`[0025] An ninth embodiment includes a cellular wireless
`communication
`system. The
`communication
`system
`includes a scheduler unit. The scheduler unit receives the
`
`protocol data units from a network and sub-dividing the
`protocol data units into sub-protocol data units. A plurality
`of base transceiver stations receive the sub-protocol data
`units, and wirelessly transmitting the sub-protocol data units
`to a subscriber unit. Time delays for transferring the sub-
`protocol data units from the scheduler unit
`to the base
`transceiver stations are estimated. The scheduler unit deter-
`
`mines a schedule protocol for transmission of the sub-
`protocol data units by the plurality of base transceiver
`stations. The schedule accounts for the time delays.
`
`[0026] Other aspects and advantages of the present inven-
`tion will become apparent
`from the following detailed
`description, taken in conjunction with the accompanying
`drawings, illustrating by way of example the principles of
`the invention.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0027] FIG. 1 shows a prior art wireless system that
`includes spatially separate transmitters.
`
`[0028] FIG. 2 shows a prior art wireless system that
`includes multiple paths from a system transmitter to a
`system receiver.
`
`[0029] FIG. 3 shows an embodiment of the invention.
`
`[0030] FIG. 4 shows another embodiment of the inven-
`tion.
`
`[0031] FIG. 5 show the time delays between the base
`station controller and the base transceiver stations of FIG. 3.
`
`[0032] FIG. 6 shows the time delays between the home
`base transceiver station and the base transceiver stations of
`FIG. 4.
`
`[0033] FIG. 7 shows an example format of a sub-protocol
`data unit.
`
`[0034] FIG. 8 shows how the example sub-protocol data
`unit of FIG. 7 can be encapsulated within an asynchronous
`transmission mode (ATM) network transmission unit.
`
`[0035] FIG. 9 shows how the example sub-protocol data
`unit of FIG. 7 can be encapsulated within an internet
`protocol (IP) network transmission unit.
`
`[0036] FIG. 10A shows a flow chart of steps included
`within an embodiment of the invention.
`
`flow chart of steps
`[0037] FIG. 10B show another
`included within another embodiment of the invention.
`
`[0038] FIG. 11A shows a set of service flow requests that
`indicate demands for data by subscriber units.
`
`[0039] FIG. 11B shows a set of estimated service flow
`buffer sizes that
`indicate demands for up link data by
`subscriber units.
`
`[0040] FIG. 12 shows a frequency spectrum of OFDM
`sub-carrier signals.
`
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`
`[0041] FIG. 13A shows a frame structure depicting blocks
`of transmission data defined by transmission time and trans-
`mission frequency.
`
`[0042] FIG. 13B shows a frame structure that includes an
`up link map transmitted at one frequency band, and a down
`link map transmitted at another frequency band.
`
`[0043] FIG. 13C shows a frame structure that include an
`up link map transmitted at a first time, and a down link map
`transmitted at a second time.
`
`[0044]
`
`FIG. 14 shows an example of a service flow table.
`
`[0045] FIG. 15 shows a flow chart of steps included
`within an embodiment of a scheduler according to the
`invention.
`
`[0046] FIG. 16 depicts several modes of block transmis-
`sion according to the invention.
`
`[0047] FIG. 17 shows a structure of a map message that
`is sent once per frame.
`
`DETAILED DESCRIPTION
`
`[0048] As shown in the drawings for purposes of illustra-
`tion, the invention is embodied in an apparatus and a method
`for scheduling wireless transmission of data blocks between
`multiple base transceiver stations and multiple receiver
`(subscriber) units. The scheduling accounts for time delays
`that exist between a scheduler unit and the base transceiver
`
`stations. The scheduling can be based on the quality of a
`transmission link between the base transceiver stations and
`
`the amount of data requested by the
`the receiver units,
`receiver units, and/or the type of data requested by the
`receiver units. The scheduling generally includes assigning
`frequency blocks and time slots to each of the receiver units
`for receiving or transmitting data blocks. The transmission
`scheduling allows for spatial multiplexing and communica-
`tion diversity through spatially separate base station trans-
`ceivers.
`
`[0049] FIG. 3 shows an embodiment of the invention. The
`embodiment includes a base station controller 310 receiving
`standard protocol data units (PDUs). The PDUs are divided
`into smaller sub-protocol data units that are stored in
`memory in the base station controller 310. The base station
`controller 310 is connected to multiple base transceiver
`stations 330, 350, 370. The base station controller 310
`includes a scheduler 316. The scheduler 316 generates a map
`that designates time slots and frequency block in which the
`sub-protocol data units are to be transmitted from the base
`transceiver stations 330, 350, 370 to receiver (subscriber)
`units 397, 399 (down link), and time slots and frequency
`blocks in which other sub-protocol data units are to be
`transmitted from the receiver (subscriber) units 397, 399 to
`the base transceiver stations 330, 350, 370 (up link).
`
`[0050] The data interface connections 355 between the
`base station controller 310 and the multiple base transceiver
`stations 330, 350, 370, are generally implemented with
`standard networking protocols because these protocol have
`been well established and adopted. The standard networking
`protocols can be, for example, asynchronous transmission
`mode (ATM) or internet protocol (IP) interconnection net-
`works. Other types of standard networking protocols can be
`used. The sub-protocol data units are not directly adaptable
`for transmission over ATM or IP networks. Therefore, the
`
`sub-protocol data units must be encapsulated within an ATM
`or IP packet switched data unit. The encapsulation process
`will be discussed later.
`
`[0051] A media access control (MAC) adaptation unit 312
`within the base station controller 310 receives the protocol
`data units from a standard computer network. The protocol
`data units can be ethernet frames, ATM cells or IP packets.
`The MAC adaptation unit 312 divides the protocol data units
`into smaller sub-protocol data units that are more adaptable
`for transmission within wireless communication systems.
`Smaller sub-protocol data units facilitate error recovery
`through retransmission.
`
`[0052] The digital circuitry required to divide or break
`large groups of data into smaller groups of data is well
`known in the art of digital circuit design.
`
`[0053] The sub-protocol data units are stored within sub-
`protocol data unit buffers 314 of the base station controller
`310. The sub-protocol data unit buffers 314 provide easy
`access to the sub-protocol data units according to a trans-
`mission schedule.
`
`[0054] A scheduler 316 generates a map or schedule of
`transmission of the sub-protocol data. This includes when
`and at what frequency range sub-protocol data units are to be
`received by the receiver (subscriber) unit 397, 399, and
`when and at what frequency range the receiver (subscriber)
`units 397, 399, transmit sub-protocol data units back to the
`base station transceivers 330, 350, 370. The map is trans-
`mitted to the receiver (subscriber) units 397, 399, so that
`each receiver (subscriber) unit knows when to receive and
`transmit sub-protocol units. A map is transmitted once per a
`unit of time that is generally referred to as a frame. The time
`duration of the frame is variable.
`
`[0055] The scheduler 316 receives information regarding
`the quality of transmission links between the base station
`transceivers 330, 350, 370 and the receiver (subscriber) units
`397, 399. The quality of the links can be used to determine
`whether the transmission of data to a particular receiver
`should include spatial multiplexing or communication diver-
`sity. Additionally, the scheduler 316 receives data requests
`from the receiver (subscriber) units. The data requests
`include information regarding the size of the data request,
`and the data type of the data request. The scheduler includes
`the link quality information, the data size, and the data type
`for generating the schedule. A detailed discussion of an
`implementation of the scheduler will follow.
`
`[0056] The scheduler 316 accesses the sub-protocol data
`units within the sub-protocol data buffers 314. A predeter-
`mined number of sub-protocol data units are retrieved by the
`scheduler 316 and ordered within frames of framing units
`332, 352, 372 within the base transceiver stations 330, 350,
`370. Amap of the schedule is include within every frame for
`the purpose of indicating to each receiver unit when and at
`what frequency data blocks requested by the receiver unit
`will be transmitted, and when and at what frequency data
`blocks can be transmitted from the receiver unit. The frame
`
`includes a predetermined number of sub-protocol data units
`as will be described later. Implementations of the framing
`units 332, 352, 372 will be discussed later.
`
`[0057] The framed sub-protocol data units are received by
`coding, diversity processing, multi-carrier modulation units
`334, 354, 374. The coding within the units 334, 354, 374 will
`
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`
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`US 2002/0055356 A1
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`
`be discussed later. The units 334, 354, 374 can include
`diversity processing of the sub-protocol units. Diversity
`communications and processing is well known in the field of
`communications.
`
`[0058] Multi-carrier modulator units 334, 354, 374 each
`generate a plurality of multiple-carrier modulated signals.
`Each multi-carrier modulator 334, 354, 374 receives a
`processed (coding and/or diversity processing) sub-protocol
`data unit stream and generates a multiple-carrier modulated
`signal based on the corresponding processed sub-protocol
`data unit stream. The multiple-carrier modulated signals are
`frequency up-converted and amplified as is well known in
`the art of communication systems.
`
`[0059] An output of a first multi-carrier modulator 334 is
`connected to a first transmit antenna 384. An output of a
`second multi-carrier modulator 354 is connected to a second
`
`transmit antenna 382. An output of a third multi-carrier
`modulator 374 is connected to a third transmit antenna 386.
`The first transmit antenna 384, the second transmit antenna
`382, and the third transmit antenna 386 can be located within
`an antenna array at a single base station. Alternatively, the
`first transmit antenna 384, the second transmit antenna 382,
`and the third transmit antenna 386 can each be located at
`
`separate base stations. The first transmit antenna 384, the
`second transmit antenna 382, and the third transmit antenna
`386 can have different polarization states. Circuitry associ-
`ated with the transmitter chains can be separately located
`with the antennas 384, 382, 386.
`
`[0060] The embodiment of FIG. 3 includes three transmit
`base transceiver stations. It
`is to be understood that the
`invention can include two or more transmit base transceiver
`
`stations. The additional antennas can be driven by additional
`multi-carrier modulators that each include separate corre-
`sponding processed sub-protocol data unit streams.
`
`[0061] The embodiment of FIG. 3 includes subscriber
`units 397, 399. The subscribers units 397, 399 can include
`multiple spatially separate subscriber antennae.
`
`and/or multiple
`[0062] Multiple transmitter antennae
`receiver antennae allow the wireless communication system
`to include spatial multiplexing and communication diversity.
`As described earlier, spatial multiplexing and communica-
`tion diversity can improve the capacity of the communica-
`tion system and reduce the effects of fading and multi-path
`resulting in increased capacity.
`
`[0063] Spatial multiplexing and diversity require sub-
`protocol data units transmitted from separate base stations
`and to be received by common receiver (subscriber) units to
`be precisely synchronized in time. That is,
`if a receiver
`(subscriber) unit is to receive sub-protocol data units from
`separate base transceiver stations, in a same frequency block
`and time slot, the base transceiver stations must be synchro-
`nized, and time delays between the base station controller
`and the base transceiver stations must be known.
`
`[0064] Timing and Synchronization of the Base Trans-
`ceiver Stations
`
`[0065] The embodiments of the invention include trans-
`mitting information from multiple base transceiver stations
`that are physically separated. As previously stated,
`the
`scheduler 316 generates a map that depicts time slots and
`frequency block in which the sub-protocol data units are to
`
`be transmitted from the base transceiver stations 330, 350,
`370 to receiver (subscriber) units 397, 399, and time slots
`and frequency blocks in which other sub-protocol data units
`are to be transmitted from the receiver (subscriber) units
`397, 399 to the base transceiver stations 330, 350, 370.
`However, because the base transceiver stations are typically
`located at different locations than the base station controller,
`a time delay generally exists between the base station
`controller and the base transceiver stations. That is, when
`sub-protocol data units are accessed from the sub-protocol
`data unit buffers for transmission from a base transceiver
`
`station, a delay will occur due to the time required to transfer
`the sub-protocol data units to the base transceiver station.
`
`In order for a multiple antenna system to properly
`[0066]
`operate, sub-protocol data units must be simultaneously
`transmitted from multiple