�f l'..� �nN�:� .. ] ... :::� u "::�
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`<•r -.,,-;_. . --··-'"
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`,,_
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`U.S. Department of Commerce
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`Patent and Trademark Office
`PATENT
`
`J ({) -2 J? -c7L__
`11111111111111111
`IIIIHIII
`IIII
`11\II
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`23696
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`PATENTTRADEMARKOFFICE
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`'P'fO/SB/16
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`
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`PROVISIONAL APPLICATION FOR PATENT COVER SHEET
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`
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`
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`This is a request for filing a PROVISIONAL APPLICATION FOR PATENT under 37 CFR l.53(c)
`
`
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`
`
`
`
`Assistant Commissioner for Patents
`
`
`Washington, D.C. 20231
`
`
`
`BOX PROVISIONAL APPLICATION
`
`Attorney Docket No.: 020554Pl
`Date: 10/25/02
`
`Express Mail Label No.: EV180330902US
`
`MIMO WLAN SYSTEM
`
`.,
`
`INVENTOR(S)
`Residence
`Family Name or Surname
`Given Name
`
`Carlisle, Massachusetts
`Walton
`l.Jay R.
`
`
`(City and either State or Foreign Country)
`
`
`(first and middle[ if any])
`I
`I
`
`Bedford, Massachusetts
`Wallace
`2. Mark
`
`Harvard, Massachusetts
`Ketchum
`3. John W.
`Ashland, Massachusetts
`Howard
`4.Steven J.
`5.
`0Additional inventors are being named on the sheet attached hereto
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`
`
`
`TITLE OF THE INVENTION (280 characters max)
`
`CORRESPONDENCE ADDRESS
`Direct all correspondence to:
`
`
`
`D Customer Number: ___ _
`QUALCOMM Incorporated
`OR [8] Firm or
`
`Attn: Patent Dept.
`Individual Name:
`
`5775 Morehouse Drive
`Address:
`
`State: California
`
`Telephone: (858) 651-4404 Zip Code: 92121-1714
`City:
`San Diego
`Fax: (858) 658-2502
`USA
`.Country:
`
`ENCLOSED APPLICATION PARTS (check all that apply)
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`[81...,Specification Number of Pages: lli l,c:;J Drawing(s) Number of Sheets: 23
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`[8]0ther (specify): An�ndix A in 37 nages· AnnPndix B in 35 pa1>es
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`METHOD OF PAYMENT OF FILING FEES FOR THIS PROVISIONAL APPLICATION FOR PATENT (check one)
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`fees or credit any overpayment to· Deposit Account Number: l 7-0026 Filing Fee Amount: $160.00
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`with an agency of the United The invention was made by an agency of the United States Government or under a contract
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`States Government.
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`[8]No 0Yes, the name of the U.S. Government Agency and the Government contract number are:
`SIGNATU�
`DATE: �10=/=2=5/�0=2 _____ _
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`TYPED or PRlN,;;,�AME/�JrRATION NO.: Byron Yafuso, Reg. No. 45,244
`TELEPHONE NO. : (858) 658-3868
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`Respectfully submitted,
`
`J
`
`Pagel of l
`
`
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`(TRANSPROV VERI 5-1/17/2001)
`
`Mercedes EX1030
`U.S. Patent No. 10,965,512
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`

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`IN THE UNITED STATES PATENT AND TRADEMARK OFFICE
`
`PATENT
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`
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`In Re Application of
`
`Walton, et al.
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`Serial No. Unknown
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`Filed: Herewith
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`)
`) For: MIMO WLAN System
`)
`)
`)
`)
`) Group Art Unit: Unknown
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`CERTIFICATE OF MAILING UNDER 37 CFR § 1.10
`
`Asst. Commissioner of Patents
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`BOX PROVISIONAL PATENT APPLICATIONS
`
`Washington, D.C. 20231
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`Dear Sir:
`
`"Express Mail" mailing label No.
`
`
`Date of Deposit
`
`EV 180330902US
`10/25/02
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`with the United papers are being deposited I hereby certify that the following
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`States Postal Service "Express Mail Post Office to Addressee" service under 37 CFR §
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`to the Asst. Commissioner of above and is addressed 1 .10 on the date indicated
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`Patents, Box Patent Applications, Washington, D.C. 20231:
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`PROVISIONAL APPLICATION IN 116 PAGES; APPENDIX A IN 37 PAGES;
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`APPENDIX BIN 35 PAGES; TRANSMITTAL LETTER IN DUPLICATE; 23 SHEETS
`OF FIGURES; AND POSTCARD
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`QUALCOMM Incorporated
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`5775 Morehouse Drive
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`San Diego, California 92121-1714
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`Telephone: (858) 845-3313
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`Facsimile: (858) 658-2502
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`\
`\
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`\
`\
`
`

`

`020554Pl
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`MIMO WLAN SYSTEM
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`Related
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`A related co-pending U.S. provisional Application entitled "Channel Calibration
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`for a Time Division Duplexed Communication System," is included in this disclosure as
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`Appendix A. A related co-pending U.S. Provisional Application entitled "Channel
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`Estimation and Spatial Processing for TDD MIMO Systems," is included in this
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`disclosure as Appendix B.
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`BACKGROUND
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`I. Field
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`[1001] The present invention
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`relates generally to data communication, and more
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`specifically to a multiple-input multiple-output (MIMO) wireless local area network
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`(WLAN) communication system.
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`II.Background
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`various are widely deployed to provide [1002] Wireless communication systems
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`types of communication such as voice, packet data, and so on. These systems may be
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`multiple-access systems capable of supporting communication with multiple users
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`sequentially or simultaneously by sharing the available system resources. Examples of
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`multiple-access systems include code division multiple access (CDMA) systems, time
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`division multiple access (TDMA) systems, and frequency division multiple access
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`(FDMA) systems.
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`to enable are also widely deployed [1003] Wireless local area networks (WLANs)
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`communication among wireless electronic devices (e.g., computers) via wireless link.
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`A WLAN may employ access points (or base stations) that act like hubs and provide
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`connectivity for the wireless devices. The access points may also connect (or "bridge")
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`the WLAN to wired LAN s, thus allowing the wireless devices access to LAN resources.
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`(RF) modulated system, a radio frequency [1004] In a wireless communication
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`signal from a transmitter unit may reach a receiver unit via a number of propagation
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`paths. The characteristics of the propagation paths typically vary over time due to a
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`number of factors, such as fading and multipath. To provide diversity against
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`deleterious path effects and improve performance, multiple transmit and receive
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`020554Pl
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`2
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`and receive paths between the transmit antennas may be used. If the propagation
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`antennas are linearly independent (i.e., a transmission on one path is not formed as a
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`true to at linear combination of the transmissions on the other paths), which is generally
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`least an extent, then the likelihood of correctly receiving a data transmission increases
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`as the number of antennas increases. Generally, diversity increases and performance
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`improves as the number of transmit and receive antennas increases.
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`[1005] A multiple-input multiple-output
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`(MIMO) communication system employs
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`antennas and multiple (NR) receive
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`antennas for data
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`multiple (NT) transmit
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`transmission. A MIMO channel formed by the NT transmit
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`and NR receive antennas
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`may be decomposed into Ns independent
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`channels, with Ns � min{Nr, NR}. Each of
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`the Ns independent
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`channels corresponds to a dimension. The MIMO system can
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`provide improved performance (e.g., increased transmission capacity and/or greater
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`reliability) if the additional
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`dimensionalities created by the multiple transmit and receive
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`antennas are utilized.
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`limited by. system are typically [1006] The resources for a given communication
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`various regulatory constraints and requirements and by other practical considerations.
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`However, the system may be required to support a number of terminals, provide various
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`services, achieve certain performance goals, and so on.
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`of [1007] There is, therefore, a need in the art for a MIMO WLAN system capable
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`supporting multiple users and providing high system performance.
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`and capabilities [1008] A multiple-access MIMO WLAN system having various
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`SUMMARY
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`able to achieve high performance is described herein. In an embodiment, the system
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`employs MIMO and orthogonal frequency division multiplexing (OFDM) to attain high
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`throughput, combat deleterious path effects, and provide other benefits.
`Each access
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`point in the system can support multiple user terminals. The allocation of downlink and
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`uplink resources is dependent on the requirements of the user terminals, the channel
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`conditions, and other factors.
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`[1009] A channel
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`structure is provided herein to support efficient downlink and
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`uplink transmissions. The channel structure comprises a number of transport channels
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`that may be used for a number of functions, such as signaling of system parameters and
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`resource assignments, downlink and uplink data transmissions, random access of the
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`system, and so on. Various attributes of these transport channels are configurable,
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`which allows the system to easily adapt to changing channel and loading conditions.
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`

`020554Pl
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`t'
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`3
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`modes are supported [1010] Multiple rates and transmission
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`system to attain high throughput when supported by the channel conditions and the
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`capabilities of the user termina1s. The rates are configurable based on estimates of the
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`channel conditions and may be independently selected
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`for the downlink and uplink.
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`Different transmission modes may also be used, depending on the number of antennas at
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`the user terminals and the channel conditions. Each transmission mode is associated
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`with different spatial processing at the transmitter and receiver and may be selected for
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`use under different operating conditions. The spatial processing facilitates data
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`transmission from multiple transmit antennas and/or data reception with multiple
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`receive antennas for higher throughput and/or diversity.
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`band frequency [1011] In an embodiment, the MIMO WLAN system uses a single
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`for both the downlink and uplink, which share the same operating band using time­
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`division duplexing (TDD). For a TDD system, the downlink and uplink channel
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`responses are reciprocal. Calibration techniques are provided herein to determine and
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`account for differences in the frequency responses of the transmit/receive chains at the
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`access point and user terminals. Techniques are also described herein to simplify the
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`spatial processing at the access point and user terminals by taking advantage of the
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`reciprocal nature of the downlink and uplink and the calibration.
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`[1012] A pilot structure is provided and includes several types of pilot used for
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`different functions. For example, beacon pilot may be used for frequency and system
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`acquisition, a MIMO pilot may be used for channel estimation, a steered reference may
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`be used for improved channel estimation, and a carrier pilot may be used for phase
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`tracking.
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`[1013] Various control loops are also provided for proper system operation. Rate
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`control may be exercised independently on the downlink and uplink.
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`may be exercised for certain transmissions (e.g., fixed-rate services). Timing control
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`may be used for uplink transmissions to account for different propagation delays of user
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`terminals located throughout the system.
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`to to allow user terminals [1014) Random access techniques are also provided
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`access the system. These techniques support system access by multiple user terminals,
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`fast acknowledgment of system access attempts, and quick assignment of
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`downlink/uplink resources.
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`are described in of the invention [1015] The various aspects and embodiments
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`further detail below.
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`020554Pl
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`4
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`will become invention of the present [1016] The features, nature, and advantages
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`more apparent
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`from the detailed description set forth below when taken in conjunction
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`with the drawings in which like reference characters identify correspondingly
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`throughout and wherein:
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`FIG. 1 shows a MIMO WLAN system;
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`FIG. 2 illustrates a layer structure that may be used for the MIMO WLAN
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`[1018]
`system;
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`[1019]
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`FIGS. 3A, 3B, and 3C illustrate a TDD-TDM frame structure, an FDD-TDM
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`frame structure, and an FDD-CDM frame structure, respectively;
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`[1020] FIG. 4 illustrates an exemplary transmission on five transport channels
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`(BCH, ·FCCH, FCH, RCH, and RACH) based on the TDD-TDM frame structure;
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`[1021] FIGS. 5A through 5G illustrate various protocol data unit (PDU) formats for
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`the five transport channels;
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`[1022]
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`FIG. 6 illustrates a structure for an FCH/RCH packet;
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`FIG. 7 is a block diagram of an access point and two user terminals in the
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`MIMO WLAN system;
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`[1024] FIGS. SA, 9A, and lOA are block diagrams of three transmitter units capable
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`of performing transmit processing for the diversity, spatial multiplexing, and beam­
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`steering modes, respectively;
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`processors [1025] FIGS. SB, 9B, and lOB are block diagrams of three TX diversity
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`capable of performing spatial processing for the diversity, spatial multiplexing, and
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`beam-steering modes, respectively,
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`FIG. SC is a block diagram of an OFDM modulator;
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`FIG. SD illustrates an OFDM symbol;
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`FIG. l lA is a bl,ock diagram of a framing unit and a scrambler within a TX
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`data processor;
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`[1029] FIG. l IB is a block diagram of an encoder and a repeat/puncture unit within
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`the TX data processor;
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`may be used TX data processor that [1030] FIG. l lC is a block diagram of another
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`for the spatial multiplexing mode;
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`[1031]
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`FIGS. 12A and 12B show a state diagram for operation of a user terminal;
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`FIG. 13 illustrates a timeline for the RACH;
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`FIGS. 14A and 14B show processes for controlling the rates of downlink and
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`uplink transmissions, respectively;
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`

`020554Pl
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`5
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`[1034]
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`FIG. 15 illustrates the operation of a power control loop; and
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`[1035]
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`FIG. 16 illustrates a process for adjusting the uplink timing of a user
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`terminal.
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`DETAILED DESCRIPTION
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`[1036]
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`The word "exemplary" is used herein to mean "serving as an example,
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`instance, or illustration." Any embodiment or design described herein as "exemplary"
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`is not necessarily to be construed as preferred or advantageous over other embodiments
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`or designs.
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`I. Overall System
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`[1037] FIG. 1 shows a MIMO WLAN system 100 that supports a number of users
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`and is capable of implementing various aspects and embodiments of the invention.
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`MIMO WLAN system 100 includes a number of access points (APs) 110 that support
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`communication for a number of user terminals (UTs) 120. For simplicity, only two
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`access points 110 are shown in FIG. 1. An access point is generally a fixed station that
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`is used for communicating with the user terminals.
`An access point may also be
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`referred to as a base station or some other terminology.
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`user throughout the system. Each [1038] User terminals 120 may be dispersed
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`terminal may be a fixed or mobile terminal that can communicate with the access point.
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`A user terminal may also be referred to as a mobile station, a remote station, an access
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`terminal, a user equipment (UE), a wireless device, or some other terminology. Each
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`user terminal may communicate with one or possibly multiple access points on the
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`downlink and/or uplink at any given moment. The downlink (i.e., forward link) refers
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`to transmission from the access point to the user terminal, and the uplink (i.e., reverse
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`link) refers to transmission from the user terminal to the access point.
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`120a through with user terminals [1039] In FIG. I, access point 110a communicates
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`120f, and access point 110b communicates with user terminals 120f through 120k.
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`Depending on the specific design of system 100, an access point may communicate with
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`multiple user terminals simultaneously (e.g., via multiple code channels or subbands) or
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`sequentially (e.g., via multiple time slots). At any given moment, a user terminal may
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`receive downlink transmissions from one or multiple access points. The downlink
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`transmission from each access point may include overhead data intended to be received
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`by multiple user terminals, user-specific data intended to be received by specific user
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`020554Pl
`
`6
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`terminals, other types of data, or any combination thereof. The overhead data may
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`include
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`pilot, page and broadcast
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`messages, system parameters, and so on.
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`(1040] The MIMO WLAN system is based on a centralized controller network
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`architecture. Thus, a system controller 130 couples to access points 110 and may
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`further couple to other systems and networks. For example, system controller 130 may
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`couple to a packet data network (PDN), a wired local area network (LAN), a wide area
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`network (WAN), the Internet, a public switched telephone network (PSTN), a cellular
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`communication network, and so on. System controller 130 may be designed to perform
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`a number of functions such as (1) coordination and control for the access points coupled
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`of (3) access and control to it, (2) routing of data among these access points,
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`communication with the user terminals served by these access points, and so on.
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`with [1041] The MIMO WLAN system may be able to provide high throughput
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`significantly greater coverage capability than conventional WLAN systems.
`The
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`MIMO WLAN system can support synchronous, asynchronous, and isochronous
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`data/voice services. The MIMO WLAN system may be designed to provide the
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`following features:
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`• High service reliability
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`• Guaranteed quality of service (QoS)
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`• High instantaneous data rates from 3 Mbps to greater than 300 Mbps
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`• High spectral efficiency of up to 24 bps/Hz
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`• Extended coverage range.
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`[1042] The MIMO WLAN system may. be operated
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`(e.g., the 2.4 GHz and 5.x GHz U-NII bands), subject to the bandwidth and emission
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`in various frequency bands.
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`constraints specific to the selected operating band. The system is designed to support
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`both indoor and outdoor deployments, with typical maximum cell size of 1 km or less.
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`The system supports fixed terminal applications, although some operating modes also
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`support portable and limited mobility operation.
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`1. MIMO, MISO, and SIMO
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`each throughout the specification, and as described [1043] In a specific embodiment
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`access point is equipped with four transmit and receive antennas for data transmission
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`and reception, where the same four antennas are used to transmit and to receive. The
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`system also supports the case where the transmit and receive antennas of the device (e.g.
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`access point, user terminal) are not shared, even though this configuration normally
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`

`020554Pl
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`7
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`provides lower performance than when the antennas are shared. The MIMO WLAN
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`with some other system may also be designed such that each access point is equipped
`number of transmit/receive antennas. Each user terminal may be equipped with a single_
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`transmit/receive antenna or multiple transmit/receive antennas for data transmission and
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`reception.
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`The number of antennas employed by each user terminal type may be ·
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`dependent on various factors such as, for example, the services to be support�d by the
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`user terminal (e.g., voice, data, or both), cost considerations, regulatory constraints,
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`safety issues, and so on.
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`user access point and multi-antenna [1044] For a given pairing of multi-antenna
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`terminal, a MIMO channel is formed by the Nr transmit antennas and NR receive
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`antennas available for use for data transmission. Different MIMO channels are formed
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`between the access point and different multi-antenna user terminals. Each MIMO
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`channel may be decomposed into Ns independent
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`channels, with N5 �min{Nr,N8
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`Each of the Ns independent
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`channel. An eigenmode normally refers to a theoretical construct, and Ns independent
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`of the MIMO channels is also referred to as an eigenmode
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`data streams may be sent orthogonaHy on the Ns eigenmodes of the MIMO channel.
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`The MIMO channel may also be viewed as including
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`Ns spatial channels that may be
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`used for data transmission. Each spatial channel may or may not correspond to an
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`eigenmode, depending on whether or not the spatial processing at the transmitter was
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`successful in orthogonalizing the data streams. For example, the data streams would be
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`transmitted on the spatial channels (and not the eigenmodes) if the transmitter has no
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`knowledge or an- imperfect estimate of the MIMO channel. For simplicity, in the
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`following description, the term eigenmode is also used to denote the case where an
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`attempt is made to orthogonalize the data stream, even though it may not be fully
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`successful due to, for example, an imperfect channel estimate.
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`the number of at the access point, [1045] For a given number of (�.g., four) antennas
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`spatial channels available for each user terminal is dependent on the number of antennas
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`employed by that user terminal and the characteristics of the wireless MIMO channel
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`that couples the access point antennas and the user terminal antennas. If a user terminal
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`is equipped with one antenna, then the four antennas at the access point and the single
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`antenna at the user terminal form a multiple-input single-output (MISO) channel for the
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`downlink and a single-input multiple-output (SIMO) channel for the uplink.
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`[1046] The MIMO WLAN system may be design ed to support a number of
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`transmission modes. Table I lists the transmission modes supported by an exemplary
`design of the MIMO WLAN system.
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`

`

`020554Pl
`
`8
`
`Table I
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`
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`Transmission modes
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`Description
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`SIMO
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`Data is transmitted from a single antenna
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`but may be received
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`by multiple antennas for receive diversity.
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`Data is redundantly transmitted from multiple transmit
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`Diversity
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`antennas and subbands to provide diversity.
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`at full channel spatial Data is transmitted on a single (best)
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`power using the phase steering information based on the
`Beam-steering
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`principal eigenmode of the MIMO channel.
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`Spatial multiplexing
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`higher spectral efficiency.
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`Data is transmitted on multiple spatial channels to achieve
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`For· simplicity, the term "diversity" refers to transmit diversity in the following
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`description, unless noted otherwise.
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`[1047] The transmission modes available for use for the downlink and uplink for
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`each user terminal are dependent on the number of antennas employed at the user
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`terminal. Table 2 lists the transmission modes available for different terminal types for
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`the downlink and uplink, assuming multiple (e.g., four) antennas at the access point.
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`Table 2
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`Downlink
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`Uplink
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`Transmission modes Single-Multi- Single-Multi-
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`antenna user antenna user antenna user antenna user
`terminal terminal terminal terminal
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`MISO/SIMO X
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`Diversity X
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`Beam-steering X
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`Spatial multiplexing
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`X
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`X
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`X
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`X
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`X
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`X
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`X
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`X
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`X
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`For the downlink, all of the transmission modes except for the spatial multiplexing
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`mode may be used for single-antenna user terminals, and all transmission modes may be
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`used for multi-antenna user terminals. For the uplink, all transmission modes may be
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`used by multi-antenna user terminals, while single-antenna user terminals use the SIMO
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`mode to transmit data from the one available antenna. Receive diversity (i.e., receiving
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`a data transmission with multiple receive antennas) may be used for the SIMO,
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`diversity, and beam-steering modes.
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`

`

`020554Pl
`
`9
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`[1048] The MIMO WLAN system may also be designed
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`to support various other
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`transmission modes, and this is within the scope of the invention. For example, a beam­
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`forming mode may be used to transmit data on a single eigenmode using both the
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`amplitude and phase information for the eigenmode
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`(instead of only the phase
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`information, which is all that is used by the beam-steering mode). As another example,
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`another spatial multiplexing mode can be defined whereby the transmitter simply
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`transmits multiple data streams from multiple transmit antennas (without any spatial
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`processing) and the receiver performs the necessary processing to isolate and recover
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`the data streams sent from the multiple transmit antennas. As yet another example, yet
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`another spatial multiplexing mode can be defined whereby the transmitter performs
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`spatial processing to attempt to orthogonalize the multiple data streams sent on the
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`multiple transmit antennas (but may not be completely successful because of an
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`imperfect channel estimate) and the receiver performs the necessary spatial processing
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`Thus, transmit antennas. to isolate and recover the data streams sent from the multiple
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`the spatial processing to transmit multiple data streams via multiple spatial channels
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`may be performed (1) at both the transmitter and receiver, (2) at only the receiver, or (3)
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`at only the transmitter.
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`with any may be designed [1049] In general, the access points and user terminals
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`number of transmit and receive antennas. For clarity, specific embodiments and designs
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`are described below whereby each access point is equipped with four transmit/receive
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`antennas, and each user terminal is equipped with four or less transmit/receive antennas.
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`2. OFDM
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`OFDM to effectively [1050] In an embodiment, the MIMO WLAN system employs
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`subbands. partition the overall system bandwidth into a number of (NF) orthogonal
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`These subbands are also referred to as frequency bins or subchannels. With OFDM,
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`each subband is associated with a respective subcarrier upon which data may be
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`modulated. For a MIMO system that uti1izes OFDM, each eigenmode of each subband
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`may be viewed as an independent transmission channel where the complex gain
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`associated »1ith each subband is effectively constant across the subband bandwidth.
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`64 orthogonal is partitioned into [1051] In an embodiment, the system bandwidth
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`subbands (i.e.,
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`NF = 64 ), which are assigned indices of -32 to +31. Of these 64
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`subbands, 48 subbands (e.g., with indices of±{ 1, ... , 6, 8, ... , 20, 22, ... , 26}) are used
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`for data, 4 subbands (e.g., with indices of ±{7, 21}) are used for pilot and possibly
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`signaling, the DC subband (with index of 0) is not used, and the remaining subbands are
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`

`

`020554Pl
`
`10
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`also not used and serve
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`as guard subbands. This OFDM subband structure is described
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`11: 802.lla and entitled "Part for IEEE Standard in further detail in a document
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`Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY)
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`Specifications: High-speed Physical Layer in the 5 GHz Band," September 1999, which
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`is publicly available and incorporated herein by reference.
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`Different numbers of
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`subbands and various other OFDM subband structures may also be implemented for the
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`MIMO WLAN system, and this is within the scope of the invention. For example, all
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`53 subbands with indices from -26 to +26 may be used for data transmission.
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`is first modulated [1052] For OFDM, the data to be transmitted on each subband
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`(i.e., symbol mapped) using a particular modulation scheme selected for use for that
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`subband. Z.eros are provided for the unused subbands. For each symbol period, the
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`modulation
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`symbols and zeros for all NF subbands are transformed to the time domain
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`using an inverse fast Fourier transform (IFFT) to obtain a transformed symbol that
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`comprises NF time-domain samples. The duration of each transformed symbol is
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`inversely related to the bandwidth of each subband. In one specific design for the
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`MIMO WLAN system, the system bandwidth is 20 MHz, NF
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`64 , the bandwidth of
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`=
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`each subband is 312.5 KHz, and the duration of each transformed symbol is 3.2 µsec.
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`to combat such as the ability [1053] OFDM can provide certain advantages,
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`frequency selective fading, which is characterized by different channel gains at different
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`frequencies of the overall system bandwidth. It is well known that frequency selective
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`fading causes inter-symbol interference (ISi), which is a phenomenon whereby each
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`symbol in a received signal acts as distortion to subsequent symbols in the received
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`signal. The ISi distortion degrades performance by impacting the ability to correctly
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`detect the received symbols. Frequency selective fading can be conveniently combated
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`with OFDM by repeating a portion of (or appending a cyclic prefix to) each transformed
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`symbol to form a corresponding OFDM symbol, which is then transmitted.
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`for each OFDM the amount to repeat) prefix (i.e., [1054] The length of the cyclic
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`symbol is dependent on the delay spread of the wireless channel. In particular, to
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`effectively combat ISi, the cyclic prefix should be longer than the maximum expected
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`delay spread for the system.
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`may be used for the of different lengths [1055] In an embodiment, cyclic prefixes
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`OFDM symbols, depending on the expected delay spread. For the specific MIMO
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`WLAN system described above, a cyclic prefix of 400 nsec (8 samples) or 800 nsec (16
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`samples) may be selected for use for the OFDM symbols. A '"short" OFDM symbol
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`uses the 400 nsec cyclic prefix and has a duration of 3.6 µsec. A "long" OFDM symbol
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`

`

`020554Pl
`
`11
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`uses the 800 nsec cyclic prefix and has a duration of 4.0 µsec. Short OFDM symbols
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`may be used if the maximum expected delay spread is 400 nsec or less, and long OFDM
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`symbols
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`may be used if the delay spread is greater than 400 nsec. Different cyclic
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`prefixes may be selected for use for different transport channels, and the cyclic prefix
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`may also be dynamically selectable, as described below. Higher system throughput may
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`be achieved by using the shorter cyclic prefix when possible, since more OFDM
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`symbols of shorter duration can be transmitted over a given fixed time interval.
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`[1056] The MIMO WLAN system may also be designed to not utilize OFDM, and
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`this is within the scope of the invention.
`
`3. Layer Structure
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`[1057] FIG. 2 illustrates a layer structure 200 that may be used for the MIMO
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`WLAN system. Layer structure 200 includes (1) applications and upper layer protocols
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`reference model of the ISO/OSI that approximately correspond to Layer 3 and higher
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`(upper layers), (2) protocols and services that correspond to Layer 2 (the link layer), and
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`(3)protocols and services that correspond to Layer 1 (the physical layer).
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`and protocols, such as applications [1058] The upper layers includes various
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`signaling services 212, data services 214, voice services 216, circuit data applications,
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`and so on. Signaling is typically provided as messages and data is typically provided as
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`packets. The services and applications in the upper layers originate and terminate
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`messages and packets according to the semantics and timing of the communication
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`protocol between the access point and the user terminal. The upper layers utilize the
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`services provided by Layer 2.
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`[1059] Layer 2 supports the delivery of messages and packets generated by the
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`upper layers. In the embodiment shown in FIG. 2, Layer 2 includes a Link Access
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`Control (LAC) sublayer 220, and a Medium Access Control (MAC) sublayer 230. The
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`LAC sublayer implements a data link protocol that provides for the correct transport and
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`delivery of messages generated by the upper layers. The LAC sublayer utilizes the
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`services provided by the