US009001774B2
`
`(12) United States Patent
`US 9,001,774 B2
`(10) Patent N0.:
`Khan
`
`(45) Date of Patent: Apr. 7, 2015
`
`(54) SYSTEM AND METHOD FOR CHANNEL
`ESTIMATION IN A DELAY DIVERSITY
`WIRELESS COMMUNICATION SYSTEM
`
`(71) Applicant: Samsung Electronics Co., Ltd,
`Suwon-si, Gyeonggi-do (KR)
`
`(72)
`
`Inventor: Farooq Khan, Allen, TX (US)
`
`(73) Assignee: Samsung Electronics Co., Ltd.,
`Suwon-Si (KR)
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl.No.: 14/078,204
`
`(22)
`
`Filed:
`
`Nov. 12, 2013
`
`(65)
`
`Prior Publication Data
`
`US 2014/0072061A1
`
`Mar. 13, 2014
`
`Related US. Application Data
`
`(63) Continuation of application No. 13/093,568, filed on
`Apr. 25, 2011, now Pat. No. 8,582,519, which is a
`continuation of application No. 11/390,125, filed on
`Mar. 27, 2006, now Pat. No. 7,953,039.
`
`(60) Provisional application No. 60/673,574, filed on Apr.
`21, 2005, provisional application No. 60/673,674,
`filed on Apr. 21, 2005, provisional application No.
`60/679,026, filed on May 9, 2005.
`
`(51)
`
`Int. Cl.
`H04 W 72/04
`H04L 27/26
`
`(2009.01)
`(2006.01)
`(Continued)
`
`(52) US. Cl.
`CPC .......... H04L 27/2601 (2013.01); H043 7/0671
`(2013.01); H043 7/0684 (2013.01); H04L
`25/022 (2013.01); H04L 25/0228 (2013.01);
`H04L 27/261 (2013.01); H04L 27/2647
`
`(2013.01), H04L 25/03955 (2013.01), H04L
`5/0048 (2013.01), H04B 7/12 (2013.01)
`(58) Field of Classification Search
`USPC ......... 370/328, 329, 332, 338, 343, 203, 208,
`370/210, 292, 480, 491, 498, 500; 375/260,
`375/267, 299
`See application file for complete search history.
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`6,219,334 B1
`6,842,487 B1
`
`4/2001 Sato et a1.
`1/2005 Larsson
`
`(Continued)
`
`FOREIGN PATENT DOCUMENTS
`
`EP
`EP
`
`0807989 A1
`1 185 048 A2
`
`11/1997
`3/2002
`
`(Continued)
`OTHER PUBLICATIONS
`Search Report dated Jan. 27, 2014 in connection with European
`Patent Application No. 067329078.
`
`Primary Examiner 7 Cong Tran
`
`(57)
`
`ABSTRACT
`
`A method of controlling downlink transmissions to a sub-
`scriber station capable of communicating with a base station
`of an orthogonal frequency division multiplexing (OFDM)
`network. The method comprises the steps of: receiving a first
`pilot signal from a first base station antenna; receiving a
`second pilot signal from a second base station antenna; and
`estimating the channel between the base station and sub-
`scriber station based on the received first and second pilot
`signals. The method also comprises determining a set of
`OFDM symbol processing parameters based on the step of
`estimating the channel and transmitting the OFDM symbol
`processing parameters to the base station. The base station
`uses the OFDM symbol processing parameters to control the
`relative gains and the relative delays ofOFDM symbols trans-
`mitted from the first and second antennas.
`
`10 Claims, 8 Drawing Sheets
`
`116
`
`1 02
`
`OFDM SYMBOL PROCESSING PARAMETER SETA
`
`
`
`
`ANT1
`ANT2
`
`
`
`
`
`
`
`
`
`PILOT 1
`
`515
`
`R
`PILOT 2
`
`505
`
`510
`
`
`
`PROCESSED OFDM SYMBOL (PARAMETER SET A)
`
`520
`
`PROCESSED OFDM SYMBOL (PARAMETER SET A)
`—._J._.__._.—
`R525
`
`
`
`
`
`
`
`APPLE 1001
`APPLE 1001
`
`

`

`US 9,001,774 B2
`
`Page 2
`
`(51)
`
`(56)
`
`Int. Cl.
`H043 ”06
`H04L 25/02
`H04L 25/03
`H04L 5/00
`H04B 7/12
`
`(2006-01)
`(200601)
`(200601)
`(2006.01)
`(2006.01)
`
`References Cited
`US. PATENT DOCUMENTS
`
`1/2006 Mohamadi
`6,982,670 B2
`2/2006 Jitsukawa et al.
`7,003,415 B2
`7/2006 Larsson ........................ 375/224
`7,082,159 B2 *
`
`4/2007 Lindskog .
`. 455/101
`7,206,554 B1*
`7/2007 Dubuc et al.
`.................. 375/296
`7,251,291 B1 *
`9/2008 Lee et al.
`7,428,267 B2
`................... 370/203
`7,453,793 B1 * 11/2008 Jones et al.
`2004/0086055 A1 *
`5/2004 Li
`................................. 375/260
`
`6/2004 Jeon et al.
`2004/0110510 A1
`2
`ton et a .
`338821 335““?
`3883/8122; 2‘1
`12/2004 Park et al.
`2004/0248527 A1
`.................. 375/343
`7/2005 Gupta et al.
`2005/0163263 A1*
`8/2005 Mantravadi et al.
`2005/0176436 A1
`9/2005 Aoki et al.
`.................... 370/208
`2005/0201268 A1*
`2005/0265477 A1* 12/2005 Takeda et al.
`................. 375/299
`
`2006/0034163 A1
`2006/0045200 A1 *
`2007/0281624 A1
`2008/0002568 A1
`
`2/2006 Gore et al.
`3/2006 Bocquet ........................ 375/267
`12/2007 Thomas et a1.
`1/2008 Wu et al.
`
`FOREIGN PATENT DOCUMENTS
`
`EP
`EP
`EP
`
`1185001 A2
`1359684 A1
`1453 223 A1
`
`3/2002
`11/2003
`9/2004
`
`* Cited by examiner
`
`

`

`U.S. Patent
`
`Apr. 7, 2015
`
`Sheet 1 0f8
`
`US 9,001,774 B2
`
`100
`
`101
`
`130
`
`\
`
`125
`
`//S’
`
`,—-—
`
`//
`
`/
`
`/
`
`/
`
`/
`'
`
`\
`\
`
`\
`
`\
`
`\
`
`\
`
`\
`
`\
`
`\\
`
`\\
`
`111
`
`112
`
`\
`
`\
`
`\
`1
`
`1I
`
`/
`
`1,.-
`
`/
`
`11
`
`3 /
`//
`
`//
`
`FIG. 1
`
`

`

`U.S. Patent
`
`Apr. 7, 2015
`
`Sheet 2 0f8
`
`US 9,001,774 B2
`
`200
`
`ANTENNAS
`
`TO
`
`ADD
`CYCUC
`PREHX
`
`OFDM
`SYMBOL
`PROC.
`
`25o
`
`CHANNEL
`
`

`

`US. Patent
`
`Apr. 7, 2015
`
`Sheet 3 of 8
`
`US 9,001,774 B2
`
`331
`
`332
`
`333
`
`230
`
`OFDM SYMBOLS
`
`(N+G SAMPLES)
`
`.93
`
`“'2‘
`
`FIG. 3
`
`

`

`U.S. Patent
`
`Apr. 7, 2015
`
`Sheet 4 0f8
`
`US 9,001,774 B2
`
`FRAME =
`
`|
`10 ms.
`|
`
`TTI
`1
`
`TTI
`2
`
`TTI
`3
`
`TTI
`4
`
`/
`
`/
`
`|
`
`/
`
`\ \
`
`\
`
`0.1667
`
`|
`
`\ \
`
`\
`
`\
`
`TTI
`12
`
`TII
`13
`
`TTI
`14
`
`TTI
`15
`
`/
`
`/
`
`|
`
`,
`
`\ \
`
`\
`
`0.1667
`
`|
`
`\\
`
`\
`
`PILOT
`PREAMB
`
`OFDM
`SYMB.
`
`OFDM
`SYMB.
`
`OFDM
`SYMB.
`
`OFDM
`SYMB.
`
`OFDM
`SYMB.
`
`OFDM
`SYMB.
`
`OFDM
`SYMB.
`
`|
`
`T“ = _..___.:
`0.667 ms.
`|
`
`L_____ TTI =
`|
`0.667 ms.
`
`I
`
`FIG. 4A
`
`410
`
`DETECT PILOT AND PERFORM CHANNEL ESTIMATION
`
`
`
`420
`
`DETERMINE DEGREE OF MULTIPATH AND FREQUENCY SELECTIVITY
`
`
`
`430
`DETERMINE OFDM SYMBOL PROCESSING PARAMETERS
`
`
`
`FEEDBACK OFDM SYMBOL PROCESSING PARAMETERS TO BASE STATION
`
`
`FIG. 4B
`
`440
`
`

`

`US. Patent
`
`Apr. 7, 2015
`
`Sheet 5 0f8
`
`US 9,001,774 B2
`
`OFDM SYMBOL PROCESSING PARAMETER SETA
`
`505
`
`510
`
`515
`
`PILOT 1
`
`PILOT 2
`
`PROCESSED OFDM SYMBOL (PARAMETER SET A)
`
`520
`PROCESSED OFDM SYMBOL (PARAMETER SET A)
`
`525
`
`I
`
`FIG. 5
`
`I
`
`l
`I
`
`I
`
`I I
`
`I
`
`I
`
`

`

`U.S. Patent
`
`Apr. 7, 2015
`
`Sheet 6 0f8
`
`US 9,001,774 B2
`
`605
`
`RECEIVE PILOT1
`FROM ANT1
`
`610
`
`RECEIVE PILOT2
`FROM ANTZ
`
`650
`
`RECEIVE OFDM
`DATA SYMBOLS
`
`615
`
`COMPENSATE PILOT1
`ACCORDING TO
`PARAMETER SET A
`
`PARAMETER SET A
`
`620
`
`COM PENSATE PILOT2
`ACCORDING TO
`
`630
`
`COMBINE
`PILOTS
`
`640
`
`ESTIMATE
`CHANNEL
`
`660
`
`DEMODULATE
`
`FIG. 6
`
`DATA
`
`

`

`US. Patent
`
`Apr. 7, 2015
`
`Sheet 7 0f8
`
`US 9,001,774 B2
`
`I
`
`I
`|
`
`I
`I
`
`I
`I
`
`|
`I
`
`'
`
`UPLINK PILOT, PREAMBLE OR DATA
`
`705
`
`710
`
`715
`
`PILOT 1
`
`PILOTZ
`
`OFDM SYMBOL PROCESSING PARAMETER SETA
`
`720
`PROCESSED OFDM SYMBOL (PARAMETER SET A)
`
`725
`PROCESSED OFDM SYMBOL (PARAMETER SET A)
`
`730
`
`FIG. 7
`
`I
`
`'
`
`

`

`U.S. Patent
`
`Apr. 7, 2015
`
`Sheet 8 0f8
`
`US 9,001,774 B2
`
`116
`
`UPLINK PILOT, PREAMBLE OR DATA
`
`
`
`
`PROCESSED PILOT 1 (PARAMETER SETA)
`
`
`
`
`810
`
`PROCESSED PILOT 2 (PARAMETER SET A)
`
`
`
`825
`
`PROCESSED OFDM SYMBOL (PARAMETER SET A)
`
`
`
`
`
`
`PROCESSED OFDM SYMBOL (PARAMETER SET A)
`
`
`FIG. 8
`
`

`

`US 9,001,774 B2
`
`1
`SYSTEM AND METHOD FOR CHANNEL
`ESTIMATION IN A DELAY DIVERSITY
`WIRELESS COMMUNICATION SYSTEM
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS AND CLAIM OF PRIORITY
`
`The present application is claims priority as a continuation
`of, and incorporates by reference, U.S. Non-Provisional
`Application Ser. No. 13/093,568 entitled “SYSTEM AND
`METHOD FOR CHANNEL ESTIMATION IN A DELAY
`DIVERSITY WIRELESS COMMUNICATION SYSTEM”
`
`filed Apr. 25, 201 1, now US. Pat. No. 8,582,519, which is a
`continuation of US. Non-Provisional Application Ser. No.
`11/390,125 entitled “SYSTEM AND METHOD FOR
`CHANNEL ESTIMATION IN A DELAY DIVERSITY
`WIRELESS COMMUNICATION SYSTEM” and filed Mar.
`
`27, 2006, now US. Pat. No. 7,953,039, to which the present
`application also claims priority and incorporates by refer-
`ence. The present application further claims priority through
`the above-identified applications to, and incorporates by ref-
`erence, US. Provisional Patent Applications Nos. 60/673,
`574 and 60/673,674, both entitled “DIVERSITY TRANS-
`MISSION IN AN OFDM WIRELESS COMMUNICATION
`
`SYSTEM” and filed Apr. 21, 2005, and to US. Provisional
`Patent Application No. 60/679,026, entitled “CHANNEL
`ESTIMATION IN A DELAY DIVERSITY WIRELESS
`
`COMMUNICATION SYSTEM” and filed May 9, 2005.
`
`TECHNICAL FIELD OF THE INVENTION
`
`The present disclosure relates generally to wireless com-
`munications and, more specifically, to an apparatus and
`method for performing channel estimation in an orthogonal
`frequency division multiplexing (OFDM) network or an
`orthogonal frequency division multiple access (OFDMA)
`network.
`
`BACKGROUND OF THE INVENTION
`
`Conventional orthogonal frequency division multiplexing
`(OFDM) networks and orthogonal frequency division mul-
`tiple access (OFDMA) network are able to improve the reli-
`ability of the channel by spreading and/or coding data traffic
`and control signals over multiple subcarriers (i.e., tones).
`However, if the channel is flat, frequency diversity cannot be
`achieved. In order to overcome this, it is possible to introduce
`artificial frequency diversity into the transmitted signal. A
`technique for artificially introducing frequency diversity into
`an OFDM environment was disclosed in US. patent applica-
`tion Ser. No. 11/327,799, filed on Jan. 6, 2006 and incorpo-
`rated by reference above. In the device disclosed in Ser. No.
`11/327,799, multiple copies of the same OFDM symbol are
`delayed by different delay values, then amplified by the same
`or different gain values, and then transmitted from different
`antennas. This artificially introduces frequency-selective fad-
`ing in the OFDM channel, thereby allowing frequency selec-
`tivity to be exploited using frequency-domain scheduling for
`low-to-medium speed mobile devices or frequency diversity
`for higher speed mobile devices.
`However, when selecting the symbol processing param-
`eters (i.e., delay values and the gain values) applied to the
`OFDM symbols, it is important to take into consideration the
`user channel type and the mobile speed. To accomplish this,
`channel estimation is performed and the symbol processing
`parameters are determined based on the channel estimates
`and mobile speed. Therefore, there is a need for improved
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`
`apparatuses and methods for performing channel estimation
`in an OFDM wireless network that artificially introduces
`frequency diversity by delaying and amplifying multiple cop-
`ies of the same OFDM symbol and then transmitting the
`delayed and amplified OFDM symbols from different trans-
`mit antennas.
`
`SUMMARY OF THE INVENTION
`
`A method of controlling downlink transmissions to a sub-
`scriber station is provided for use in a subscriber station
`capable of communicating with a base station of an orthogo-
`nal frequency division multiplexing (OFDM) network. The
`method comprises the steps of: receiving a first pilot signal
`from a first antenna of the base station; receiving a second
`pilot signal from a second antenna of the base station; esti-
`mating the channel between the base station and subscriber
`station based on the received first and second pilot signals;
`determining a set of OFDM symbol processing parameters
`based on the step of estimating the channel, wherein the
`OFDM symbol processing parameters are usable by the base
`station to control the relative gains and the relative delays of
`OFDM symbols transmitted from the first and second anten-
`nas; and transmitting the OFDM symbol processing param-
`eter set to the base station.
`
`According to another embodiment of the present disclo-
`sure, a subscriber station capable of communicating with a
`base station of an orthogonal frequency division multiplexing
`(OFDM) network is provided. The subscriber station com-
`prises: receive path circuitry capable of receiving a first pilot
`signal from a first antenna of the base station and receiving a
`second pilot signal from a second antenna of the base station;
`and channel estimating circuitry capable of estimating the
`channel between the base station and subscriber station based
`on the received first and second pilot signals and capable of
`determining a set of OFDM symbol processing parameters
`based on a channel quality estimate. The OFDM symbol
`processing parameters are usable by the base station to con-
`trol the relative gains and the relative delays of OFDM sym-
`bols transmitted from the first and second antennas and
`
`wherein the subscriber station is capable of transmitting the
`OFDM symbol processing parameters to the base station.
`According to yet another embodiment of the present dis-
`closure, a base station is provided for use in an orthogonal
`frequency division multiplexing (OFDM) network. The base
`station comprises: 1) receive path circuitry capable of receiv-
`ing an uplink signal from a subscriber station, estimating the
`channel between the base station and subscriber station based
`
`on the received uplink signal, and determining a set of OFDM
`symbol processing parameters based on a channel quality
`estimate; and 2) transmit path circuitry capable of using the
`OFDM symbol processing parameters to control the relative
`gains and the relative delays of processed OFDM symbols
`transmitted from a first antenna and a second antenna of the
`
`base station. The base station is capable of transmitting the
`OFDM symbol processing parameters to the subscriber sta-
`tion. The OFDM symbol processing parameters are based on
`the multipath characteristics and the frequency selectivity
`characteristics of the channel.
`
`Before undertaking the DETAILED DESCRIPTION OF
`THE INVENTION below, it may be advantageous to set forth
`definitions of certain words and phrases used throughout this
`patent document: the terms “include” and “comprise,” as well
`as derivatives thereof, mean inclusion without limitation; the
`term “or,” is inclusive, meaning and/or; the term “each”
`means every one ofat least a subset ofthe identified items; the
`phrases “associated wit ” and “associated therewith,” as well
`
`

`

`US 9,001,774 B2
`
`3
`as derivatives thereof, may mean to include, be included
`within, interconnect with, contain, be contained within, con-
`nect to or with, couple to or with, be communicable with,
`cooperate with, interleave, juxtapose, be proximate to, be
`bound to or with, have, have a property of, or the like; and the
`term “controller” means any device, system or part thereof
`that controls at least one operation, such a device may be
`implemented in hardware, firmware or software, or some
`combination ofat least two ofthe same. It shouldbe noted that
`
`the functionality associated with any particular controller
`may be centralized or distributed, whether
`locally or
`remotely. Definitions for certain words and phrases are pro-
`vided throughout this patent document, those ofordinary skill
`in the art should understand that
`in many, if not most
`instances, such definitions apply to prior, as well as future
`uses of such defined words and phrases.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`For a more complete understanding of the present disclo-
`sure and its advantages, reference is now made to the follow-
`ing description taken in conjunction with the accompanying
`drawings, in which like reference numerals represent like
`parts:
`FIG. 1 illustrates an exemplary orthogonal frequency divi-
`sion multiplexing (OFDM) wireless network that is capable
`of performing channel estimation according to the principles
`of the present disclosure;
`FIG. 2A is a high-level diagram of the orthogonal fre-
`quency division multiplexing (OFDM) transmit path in a base
`station according to one embodiment of the disclosure;
`FIG. 2B is a high-level diagram of the orthogonal fre-
`quency division multiplexing (OFDM) receive path in a sub-
`scriber station according to one embodiment of the disclo-
`sure;
`FIG. 3 illustrates the OFDM symbol processing block in
`the base station in greater detail according to an exemplary
`embodiment of the present disclosure;
`FIG. 4A illustrates data traffic transmitted in the downlink
`
`from a base station to a subscriber station according to an
`exemplary embodiment of the present disclosure;
`FIG. 4B is a flow diagram illustrating the determination of
`the user channel type based on the measurements on the
`preamble according to an exemplary embodiment of the dis-
`closure;
`FIG. 5 is a message flow diagram illustrating the transmis-
`sion of OFDM symbols from a base station to a subscriber
`station according to the principles of the disclosure;
`FIG. 6 is a flow diagram illustrating the processing of pilot
`signals and OFDM data symbols according to an exemplary
`embodiment of the present disclosure;
`FIG. 7 is a message flow diagram illustrating the transmis-
`sion of OFDM symbols from a base station to a subscriber
`station according to another embodiment of the disclosure;
`and
`
`FIG. 8 is a message flow diagram illustrating the transmis-
`sion of OFDM symbols from a base station to a subscriber
`station according to another embodiment of the disclosure.
`
`DETAILED DESCRIPTION OF THE INVENTION
`
`FIGS. 1 through 8, discussed below, and the various
`embodiments used to describe the principles of the present
`disclosure in this patent document are by way of illustration
`only and should not be construed in any way to limit the scope
`of the disclosure. Those skilled in the art will understand that
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`4
`
`the principles of the present disclosure may be implemented
`in any suitably arranged wireless network.
`The present disclosure is directed to apparatuses and algo-
`rithms for channel estimation and channel quality estimation
`for demodulation and data rate selection in an orthogonal
`frequency division multiplexing (OFDM) wireless network
`that uses delayed diversity. Such a delayed diversity wireless
`network was disclosed previously US. patent application Ser.
`No. 11/327,799,
`incorporated by reference above. The
`present disclosure uses a number of factors, including user
`channel type and mobile speed, to select OFDM symbol
`processing parameters (i.e., delays D1, D2, .
`.
`.
`, DP and gains
`g0, g1, .
`.
`.
`, gP) for OFDM symbols transmitted from up to P
`antennas (i.e., ANT1 to ANTP). Therefore, different OFDM
`symbol processing parameters may be used to transmit to
`different mobile devices that are scheduled simultaneously,
`depending upon their channel types.
`It is noted that the scope of the present disclosure is not
`limited to orthogonal
`frequency division multiplexing
`(OFDM) wireless networks. The present disclosure is also
`applicable to orthogonal frequency division multiple access
`(OFDMA) wireless networks. However, for simplicity and
`brevity, the embodiments described below are directed to
`OFDM wireless networks, except where otherwise noted or
`where the context indicates otherwise.
`
`For relatively low-speed mobile devices, it is usually pos-
`sible to track changes in the channel, thereby allowing chan-
`nel sensitive scheduling to improve performance. Thus, the
`OFDM symbol processing parameters may be selected in
`such a way that relatively large coherence bandwidth results.
`That is, a relatively larger number of subcarriers experience
`similar fading. This goal may be achieved by keeping the
`delays for OFDM symbols from different antennas relatively
`small. A mobile device may then be scheduled on a subband
`consisting of contiguous subcarriers.
`For relatively high-speed mobile devices, channel quality
`variations cannot be tracked accurately, so that frequency-
`diversity may be helpful. Thus, the OFDM symbol processing
`parameters are selected in such a way that relatively small
`coherence bandwidth results. That is, potentially independent
`fading may occur from subcarrier to subcarrier. This goal may
`be achieved by having relatively large delays for OFDM
`symbols transmitted from different antennas.
`The symbol processing parameters may also be selected
`based on the degree of frequency-selectivity already present
`in the channel. For example, if a channel already has a lot of
`multipath effects and is, therefore, frequency selective, there
`may be little or no need for additional frequency selectivity.
`The OFDM symbol processing parameters may be selected
`on a user-by-user basis because different mobile devices
`experience different channel types.
`FIG. 1 illustrates exemplary orthogonal frequency division
`multiplexing (OFDM) wireless network 100, which is
`capable of performing channel estimation according to the
`principles ofthe present disclosure. In the illustrated embodi-
`ment, wireless network 100 includes base station (BS) 101,
`base station (BS) 102, base station (BS) 103, and other similar
`base stations (not shown). Base station 101 is in communica-
`tion with base station 102 and base station 103. Base station
`101 is also in communication with Internet 130 or a similar
`
`IP-based network (not shown).
`Base station 102 provides wireless broadband access (via
`base station 101) to Internet 130 to a first plurality of sub-
`scriber stations within coverage area 120 of base station 102.
`The first plurality of subscriber stations includes subscriber
`station 111, which may be located in a small business (SB),
`subscriber station 112, which may be located in an enterprise
`
`

`

`US 9,001,774 B2
`
`5
`(E), subscriber station 113, which may be located in a WiFi
`hotspot (HS), subscriber station 114, which may be located in
`a first residence (R), subscriber station 115, which may be
`located in a second residence (R), and subscriber station 116,
`which may be a mobile device (M), such as a cell phone, a
`wireless laptop, a wireless PDA, or the like.
`Base station 103 provides wireless broadband access (Via
`base station 101) to Internet 130 to a second plurality of
`subscriber stations within coverage area 125 of base station
`103. The second plurality of subscriber stations includes sub-
`scriber station 115 and subscriber station 116. In an exem-
`
`plary embodiment, base stations 101-103 may communicate
`with each other and with subscriber stations 111-116 using
`OFDM or OFDMA techniques.
`Base station 101 may be in communication with either a
`greater number or a lesser number of base stations. Further-
`more, while only six subscriber stations are depicted in FIG.
`1, it is understood that wireless network 100 may provide
`wireless broadband access to additional subscriber stations. It
`is noted that subscriber station 115 and subscriber station 116
`
`are located on the edges of both coverage area 120 and cov-
`erage area 125. Subscriber station 115 and subscriber station
`116 each communicate with both base station 102 and base
`
`5
`
`10
`
`15
`
`20
`
`station 103 and may be said to be operating in handoff mode,
`as known to those of skill in the art.
`
`25
`
`Subscriber stations 111-116 may access voice, data, video,
`video conferencing, and/or other broadband services via
`Internet 130. In an exemplary embodiment, one or more of
`subscriber stations 111-116 may be associated with an access
`point (AP) of a WiFi WLAN. Subscriber station 116 may be
`any of a number of mobile devices, including a wireless-
`enabled laptop computer, personal data assistant, notebook,
`handheld device, or other wireless-enabled device. Sub-
`scriber stations 114 and 115 may be, for example, a wireless-
`enabled personal computer (PC), a laptop computer, a gate-
`way, or another device.
`FIG. 2A is a high-level diagram of the transmit path in
`orthogonal frequency division multiplexing (OFDM) trans-
`mitter 200 according to an exemplary embodiment of the
`disclosure. FIG. 2B is a high-level diagram ofthe receive path
`in orthogonal
`frequency division multiplexing (OFDM)
`receiver 260 according to an exemplary embodiment of the
`disclosure. OFDM transmitter 200 comprises quadrature
`amplitude modulation (QAM) modulator 205, serial-to-par-
`allel (S-to-P) block 210, Inverse Fast Fourier Transform
`(IFFT) block 215, parallel-to-serial (P-to-S) block 220, add
`cyclic prefix block 225, and OFDM symbol processing block
`230. OFDM receiver 250 comprises remove cyclic prefix
`block 260, serial-to-parallel (S-to-P) block 265, Fast Fourier
`Transform (FFT) block 270, parallel-to-serial (P-to-S) block
`275, quadrature amplitude modulation (QAM) demodulator
`280, and channel estimation block 285.
`At least some of the components in FIGS. 2A and 2B may
`be implemented in software while other components may be
`implemented by configurable hardware or a mixture of soft-
`ware and configurable hardware. In particular, it is noted that
`the FFT blocks and the IFFT blocks described in FIGS. 2A
`
`and 2B may be implemented as configurable software algo-
`rithms, where the values of FFT and IFFT sizes may be
`modified according to the implementation.
`QAM modulator 205 receives a stream of input data and
`modulates the input bits (or symbols) to produce a sequence
`of frequency-domain modulation symbols. Serial-to-parallel
`block 210 converts (i.e., de-multiplexes) the serial QAM
`symbols to parallel data to produce M parallel symbol
`streams where M is the IFFT/FFT size used in OFDM trans-
`mitter 200 and OFDM receiver 250. IFFT block 215 then
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`6
`performs an IFFT operation on the M parallel symbol streams
`to produce time-domain output signals. Parallel-to-serial
`block 220 converts (i.e., multiplexes) the parallel time-do-
`main output symbols from IFFT block 215 to produce a serial
`time-domain signal.
`Add cyclic prefix block 225 then inserts a cyclic prefix to
`each OFDM symbol in the time-domain signal. As is well
`known, the cyclic prefix is generated by copying the last G
`samples of an N sample OFDM symbol and appending the
`copied G samples to the front of the OFDM symbol. Finally,
`OFDM symbol processing block 230 processes the incoming
`OFDM symbols as described in FIG. 3 and as described in
`US. patent application Ser. No. 11/327,799. The process
`OFDM samples at the output of OFDM symbol processing
`block 23 0 are then sent to up-conversion circuitry (not shown)
`prior to being transmitted from multiples transmit antennas.
`The transmitted RF signal arrives at OFDM receiver 250
`after passing through the wireless channel and reverse opera-
`tions to those in OFDM transmitter 200 are performed.
`Remove cyclic prefix block 260 removes the cyclic prefix to
`produce the serial time-domain baseband signal. Serial-to-
`parallel block 265 converts the time-domain baseband signal
`to parallel time domain signals. FFT block 270 then performs
`an FFT algorithm to produce M parallel frequency-domain
`signals. Parallel-to-serial block 275 converts the parallel fre-
`quency-domain signals to a sequence of QAM data symbols.
`QAM demodulator 280 then demodulates the QAM symbols
`to recover the original input data stream. Channel estimation
`block 285 also receives the QAM data symbols from parallel-
`to-serial block 275 and performs channels estimation. As will
`be described below in greater detail, the channel estimation
`values are used to determine a parameter set of gain values
`and delay values that are used in OFDM symbol processing
`block 230 in OFDM transmitter 200 and are used by QAM
`demodulator 280 to demodulate the QAM data symbols.
`The exemplary transmit path of OFDM transmitter 200
`may be representative ofthe transmit paths of any one of base
`stations 101-103 or any one of subscriber stations 111-116.
`Similarly, the exemplary receive path of OFDM receiver 250
`may be representative ofthe transmit paths of any one of base
`stations 101-103 or any one of subscriber stations 111-116.
`However, since multiple antenna configurations are more
`common in base stations than in subscriber stations or other
`
`mobile devices, for the sake of simplicity and clarity, the
`descriptions that follow will be directed toward transactions
`between a base station (e.g., BS 102) that implements a trans-
`mit path similar to OFDM transmitter 200 and a subscriber
`station (e.g., SS 116) that implements a receive path similar to
`OFDM receiver 250. However, such an exemplary embodi-
`ment should not be construed to limit the scope of the present
`disclosure. It will be appreciated by those skilled in the art that
`in cases where multiple antennas are implemented in a sub-
`scriber station, the transmit path and the receiver path of both
`the base station and the subscriber station may be imple-
`mented as in shown in FIGS. 2A and 2B.
`
`FIG. 3 illustrates OFDM symbol processing block 230 in
`greater detail according to an exemplary embodiment of the
`present disclosure. OFDM symbol processing block 230
`comprises P delay elements, including exemplary delay ele-
`ments 311 and 312, P+1 amplifiers, including exemplary
`amplifiers 321, 322 and 323, and P+1 transmit antennas,
`including exemplary antennas 331, 332 and 333. Delay ele-
`ments 311 and 312 are arbitrarily labeled “D1” and “DP”,
`respectively. OFDM symbol processing block 230 receives
`incoming OFDM symbols and forwards P+1 copies of each
`OFDM symbol to the P+1 transmit antennas. Each OFDM
`symbol comprises N+G samples, where N is the number of
`
`

`

`US 9,001,774 B2
`
`7
`samples in the original data symbol and G is the number of
`samples in the cyclic prefix appended to the original symbol.
`A first copy of each OFDM symbol is applied directly to
`the input of amplifier 321, amplified by a gain value, g0, and
`sent to antenna 331.A second copy of each OFDM symbol is
`delayed by delay element 311, applied to the input of ampli-
`fier 322, amplified by a gain value, g1, and sent to antenna
`332. Other copies of each OFDM symbol are similarly
`delayed and amplified according to the number of antennas.
`By way of example, the P+1 copy of each OFDM symbol is
`delayed by delay element 312, applied to the input of ampli-
`fier 323, amplified by a gain value, gP, and sent to antenna
`333. The gain values and the delay values are determined by
`the values in an OFDM symbol processing parameter set, as
`described hereafter and in US. patent application Ser. No.
`1 1/327,799. The result is that multiple copies of each OFDM
`are transmitted, wherein each copy of an OFDM symbol is
`amplified by a selected amount and delayed by a selected
`amount relative to other OFDM symbol copies. US. patent
`application Ser. No. 11/327,799, incorporated by reference
`above, describes a number ofarchitectures for OFDM symbol
`processing block 230 that achieve such a result. In an advan-
`tageous embodiment, the delays introduced by OFDM sym-
`bol processing block 230 are cyclic delays, as disclosed in
`US. patent application Ser. No. 11/327,799.
`FIG. 4A illustrates data traffic transmitted in the downlink
`
`from base station 102 to subscriber station 116 according to
`an exemplary embodiment of the present disclosure. An
`exemplary frame of OFDM data is 10 milliseconds in length
`and comprises fifteen (15) transmit time intervals (TTIs),
`namely TTI 1 through TTI 15, where each one of TTI 1
`through TTI 15 is 0.667 milliseconds in duration. Within each
`of TTI 2 through TT1 15, there are four OFDM data symbols,
`where each OFDM data symbol is 0.1667 milliseconds in
`duration. In the first TTI, namely TTI 1 , there are three OFDM
`data symbols that are preceded by a pilot preamble symbol.
`The pilot preamble symbol is used by SS 116 to perform
`synchronization channel estimation and to determine the
`OFDM symbol processing parameter set.
`FIG. 4B is a flow diagram illustrating the determination of
`the user channel type based on the measurements on the
`preamble according to an exemplary embodiment of the dis-
`closure. In an OFDM system, a known pilot sequence is
`transmitted for one or more OFDM symbol durations. Chan-
`nel estimation block 285 in the receiver (i.e., SS 116) detects
`the known pilot signal, which is then use to perform synchro-
`nization (process step 410). Channel estimation block 285
`uses the detected preamble symbols to determine the degree
`of multipath effects in the channel and, therefore, the fre-
`quency selectivity in the channel between BS 102 and SS 116
`(process step 420).
`Based on the profile of the channel, channel estimation
`block 285 (or another processing element or controller in SS
`116) determines (i.e., calculates) a set of OFDM symbol
`processing parameters (i.e., gain values and delay values) that
`may be used in BS 102 to improve reception of OFDM
`symbols in SS 116 (process step 430). SS 116 then feeds back
`the OFDM symbol processing parameter set to BS 102 in the
`uplink (process step 440). Other factors, such as mobile
`speed, can also be used in determining (or calculating) the
`OFDM symbol processing parameters. The channel type may
`also be determined by using other mechanisms, such as ref-
`erence in time-frequency.
`In this manner, BS 102 receives an OFDM symbol process-
`ing parameter set from each subscriber station. Thereafter, as
`BS 102 schedules each subscriber station to receive data, BS
`102 uses the OFDM symbol processing parameter set for that
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`8
`subscriber station to modify the OFDM symbols transmitted
`from each antenna for BS 102. For example, BS 102 may use
`OFDM Symbol Processing Parameter Set A to transmit
`OFDM symbols from two or more antennas to SS 116 and
`may use OFDM Symbol Processing Parameter Set B to
`simultaneously transmit OFDM symbols from t