(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2004/0179627 A1
`(43) Pub. Date:
`Sep. 16, 2004
`Ketchum et al.
`
`US 2004O179627A1
`
`(54)
`
`PILOTS FOR MIMO COMMUNICATION
`SYSTEMS
`
`(76) Inventors: John W. Ketchum, Harvard, MA (US);
`Mark S. Wallace, Bedford, MA (US);
`J. Rodney Walton, Carlisle, MA (US);
`Steven J. Howard, Ashland, MA (US)
`Correspondence Address:
`Qualcomm Incorporated
`Patents Department
`5775 Morehouse Drive
`San Diego, CA 92.121-1714 (US)
`Appl. No.:
`10/610,446
`
`(21)
`(22)
`
`Filed:
`
`Jun. 30, 2003
`Related U.S. Application Data
`(60) Provisional application No. 60/421,309, filed on Oct.
`25, 2002. Provisional application No. 60/421,462,
`filed on Oct. 25, 2002. Provisional application No.
`60/421,428, filed on Oct. 25, 2002. Provisional appli
`cation No. 60/432,617, filed on Dec. 10, 2002. Pro
`visional application No. 60/438,601, filed on Jan. 7,
`2003.
`
`Publication Classification
`
`(51) Int. Cl. ................................................... H04L 1/02
`(52) U.S. Cl. .............................................................. 375/267
`
`(57)
`
`ABSTRACT
`
`Pilots suitable for use in MIMO systems and capable of
`Supporting various functions are described. The various
`types of pilot include-a beacon pilot, a MIMO pilot, a
`Steered reference or Steered pilot, and a carrier pilot. The
`beacon pilot is transmitted from all transmit antennas and
`may be used for timing and frequency acquisition. The
`MIMO pilot is transmitted from all transmit antennas but is
`covered with different orthogonal codes assigned to the
`transmit antennas. The MIMO pilot may be used for channel
`estimation. The Steered reference is transmitted on Specific
`eigenmodes of a MIMO channel and is user terminal spe
`cific. The Steered reference may be used for channel esti
`mation. The carrier pilot may be transmitted on designated
`Subbands/antennas and may be used for phase tracking of a
`carrier Signal. Various pilot transmission Schemes may be
`devised based on different combinations of these various
`types of pilot.
`
`
`
`Transmit a beacon pilot
`and a MIMO pilot on the
`downlink in each TD) frame
`
`Receive the downlink
`beacon and MEMO pilots
`and acquire the system
`
`Obtain estimate of uplink
`channebased on received
`uplink MIMO pilot
`
`Obtain estimates of pertinent
`matrices based on received
`uplink steered reference
`
`336
`
`Transmit a steered
`reference on the downlink
`for data transmission
`
`Obtain estimate of downlink
`channelbased on received
`downlink MIMO pilot
`
`l
`
`Transmit a steered
`reference on the uplink
`for data transmission
`
`Update estimate of downlink
`channelbased on received
`downlink MIMO pilot and
`pertinent matrices based on
`steered reference (if any)
`
`Transmit a carrier pilot
`on the downlink for
`data transmission
`
`Track the phase of downlink
`carrier signal based on
`received downlink carrier pilot
`
`Track the phase of uplink
`carrier signal based on
`received uplink carrier pilot
`
`Transmit a carrier pilot
`on the uplink for data
`transmission
`
`VWGoA EX1006
`U.S. Patent No. 10,965,512
`
`

`

`Patent Application Publication Sep. 16, 2004 Sheet 1 of 9
`
`US 2004/0179627 A1
`
`
`
`

`

`Patent Application Publication Sep. 16
`
`9
`
`2004 Sheet 2 of 9
`
`US 2004/0179627 A1
`
`003
`
`
`
`
`
`
`
`
`
`
`
`

`

`Patent Application Publication Sep. 16, 2004 Sheet 3 of 9
`
`US 2004/0179627 A1
`
`Access Point
`
`User T rminal
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`32
`Transmit a beacon pilot
`and a MIMO pilot on the
`downlink in each TDD frame
`
`Obtain estimate of uplink
`channel based on received
`uplink MIMO pilot
`
`
`
`
`
`Obtain estimates of pertinent
`matrices based on received
`uplink steered reference
`
`Transmit a steered
`reference on the downlink
`for data transmission
`
`
`
`Transmit a carrier pilot
`On the downlink for
`data transmission
`
`Track the phase of uplink
`carrier signal based on
`received uplink carrier pilot
`
`300
`
`alo
`
`
`
`!-----------------
`314
`Receive the downlink
`beacon and MIMO pilots
`and acquire the system
`
`Obtain estimate of downlink
`channel based on received
`downlink MIMO pilot
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`Transmit a steered
`reference on the uplink
`for data transmission
`
`Update estimate of downlink
`channel based on received
`downlink MIMO pilot and
`pertinent matrices based on
`steered reference (if any)
`
`Track the phase of downlink
`carrier signal based on
`received downlink carrier pilot
`
`Transmit a carrier pilot
`on the uplink for data
`transmission
`
`P
`
`P
`
`up em mo up
`
`o
`
`o
`
`r
`
`w
`
`me
`
`o
`
`o
`
`o
`
`op -
`
`

`

`Patent Application Publication Sep. 16, 2004 Sheet 4 of 9
`
`US 2004/0179627 A1
`
`e?eC]
`
`90.InOS
`
`
`
`
`
`e?BO XH
`
`JOSS30OJE
`
`
`
`
`
`
`
`eqeq
`
`eo/noS
`
`
`
`

`

`Patent Application Publication Sep. 16, 2004 Sheet 5 of 9
`
`US 2004/0179627 A1
`
`420a
`
`Beacon Pilot
`Subband Processor
`
`
`
`b(1)
`
`bk (k)
`
`FIG. 5
`
`for
`Antenna 1
`
`for
`Antenna 2
`
`for
`Antenna 3
`
`for
`Antenna 4
`
`

`

`Patent Application Publication Sep. 16, 2004 Sheet 6 of 9
`
`US 2004/0179627 A1
`
`
`
`420b
`
`for
`Antenna 1
`
`for
`Antenna 2
`
`for
`Antenna 3
`
`for
`Antenna 4
`
`p(k)
`
`FIG. 6A
`
`

`

`Patent Application Publication Sep. 16, 2004 Sheet 7 of 9
`
`US 2004/0179627 A1
`
`460b
`
`454a
`
`454b
`
`454C
`
`454d
`
`(k)
`
`f(k)
`
`I,(k)
`
`
`
`
`
`MIMO Pilot
`Subband/Antenna Processor
`
`660c
`
`MIMO Pilot
`Subband/Antenna Processor
`
`660
`
`MMO Pilot
`Subband/Antenna Processor
`
`W
`h(k)
`ha(k)
`ha(k)
`h()
`
`A.
`h(k)
`ha(k)
`h()
`h.(R)
`
`FIG. 6B
`
`

`

`Patent Application Publication Sep. 16, 2004 Sheet 8 of 9
`
`US 2004/0179627 A1
`
`420C
`
`Steered Reference
`Subband Processor
`
`
`
`p(1)
`
`p(k)
`
`FIG. 7A
`
`for
`Antenna 1
`
`for
`Antenna 2
`
`for
`Antenna 3
`
`for
`Antenna 4
`
`

`

`Patent Application Publication Sep. 16, 2004 Sheet 9 of 9
`
`US 2004/0179627 A1
`
`460c
`
`750k
`
`
`
`454a
`
`DEMOD r(k)
`
`454b
`
`454C
`DEMOD 39
`
`454d
`
`

`

`US 2004/0179627 A1
`
`Sep. 16, 2004
`
`PILOTS FOR MIMO COMMUNICATION SYSTEMS
`0001. This application claims the benefit of provisional
`U.S. Application Serial No. 60/421,309, entitled “MIMO
`WLAN System,” filed on Oct. 25, 2002, Serial No. 60/421,
`462, entitled “Channel Calibration for a Time Division
`Duplexed Communication System,” filed on Oct. 25, 2002,
`Serial No. 60/421,428, entitled “Channel Estimation and
`Spatial Processing for TDD MIMO Systems," filed on Oct.
`25, 2002, and Serial No. 60/438,601, entitled “Pilots for
`MIMO Communication Systems,” filed on Jan. 7, 2003, all
`of which are assigned to the assignee of the present appli
`cation and incorporated herein by reference in their entirety
`for all purposes.
`
`BACKGROUND
`
`0002 I. Field
`0003. The present invention relates generally to data
`communication, and more specifically to pilots Suitable for
`use in multiple-input multiple-output (MIMO) communica
`tion Systems.
`0004 II. Background
`0005) A MIMO system employs multiple (N) transmit
`antennas and multiple (NR) receive antennas for data trans
`mission. A MIMO channel formed by the N transmit and
`N, receive antennas may be decomposed into Ns indepen
`dent channels, which are also referred to as eigenmodes,
`where Nss min{NT, N. Each of the Ns independent
`channels corresponds to a dimension. The MIMO system
`can provide improved performance (e.g., increased trans
`mission capacity and/or greater reliability) if the additional
`dimensionalities created by the multiple transmit and receive
`antennas are utilized.
`0006. In a wireless communication system, data to be
`transmitted is first modulated onto a radio frequency (RF)
`carrier Signal to generate an RF modulated Signal that is
`more Suitable for transmission over a wireleSS channel. For
`a MIMO system, up to N RF modulated signals may be
`generated and transmitted Simultaneously from the N trans
`mit antennas. The transmitted RF modulated Signals may
`reach the N receive antennas via a number of propagation
`paths in the wireleSS channel. The characteristics of the
`propagation paths typically vary over time due to a number
`of factorS Such as, for example, fading, multipath, and
`external interference. Consequently, the transmitted RF
`modulated Signals may experience different channel condi
`tions (e.g., different fading and multipath effects) and may
`be associated with different complex gains and Signal-to
`noise ratios (SNRs).
`0007 To achieve high performance, it is often necessary
`to characterize the response of the wireleSS channel. For
`example, the channel response may be needed by the trans
`mitter to perform spatial processing (described below) for
`data transmission to the receiver. The channel response may
`also be needed by the receiver to perform Spatial processing
`on the received signals to recover the transmitted data.
`0008. In many wireless communication systems, a pilot is
`transmitted by the transmitter to assist the receiver in
`performing a number of functions. The pilot is typically
`generated based on known Symbols and processed in a
`
`known manner. The pilot may be used by the receiver for
`channel estimation, timing and frequency acquisition, data
`demodulation, and So on.
`0009 Various challenges are encountered in the design of
`a pilot structure for a MIMO system. As one consideration,
`the pilot Structure needs to address the additional dimen
`Sionalities created by the multiple transmit and multiple
`receive antennas. AS another consideration, Since pilot trans
`mission represents overhead in the MIMO system, it is
`desirable to minimize pilot transmission to the extent poS
`sible. Moreover, if the MIMO system is a multiple-access
`System that Supports communication with multiple users,
`then the pilot Structure needs to be designed Such that the
`pilots needed to Support the multiple users do not consume
`a large portion of the available System resources.
`0010. There is therefore a need in the art for pilots for
`MIMO systems that address the above considerations.
`
`SUMMARY
`0011 Pilots suitable for use in MIMO systems are pro
`Vided herein. These pilots can Support various functions that
`may be needed for proper System operation, Such as timing
`and frequency acquisition, channel estimation, calibration,
`and So on. The pilots may be considered as being of different
`types that are designed and used for different functions.
`0012. The various types of pilot may include: a beacon
`pilot, a MIMO pilot, a steered reference or steered pilot, and
`a carrier pilot. The beacon pilot is transmitted from all
`transmit antennas and may be used for timing and frequency
`acquisition. The MIMO pilot is also transmitted from all
`transmit antennas but is covered with different orthogonal
`codes assigned to the transmit antennas. The MIMO pilot
`may be used for channel estimation. The Steered reference is
`transmitted on specific eigenmodes of a MIMO channel and
`is user terminal Specific. The Steered reference may be used
`for channel estimation and possibly rate control. The carrier
`pilot may be transmitted on certain designated Subbands/
`antennas and may be used for phase tracking of a carrier
`Signal.
`0013 Various pilot transmission schemes may be devised
`based on different combinations of these various types of
`pilot. For example, on the downlink, an access point may
`transmit a beacon pilot, a MIMO pilot, and a carrier pilot for
`all user terminals within its coverage area and may option
`ally transmit a steered reference to any active user terminal
`that is receiving a downlink transmission from the access
`point. On the uplink, a user terminal may transmit a MIMO
`pilot for calibration and may transmit a steered reference and
`a carrier pilot when Scheduled (e.g., for downlink and/or
`uplink data transmissions). The processing to transmit and
`receive these various types of pilot is described in further
`detail below.
`0014 Various aspects and embodiments of the invention
`are also described in further detail below.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`0015 The features and nature of the present invention
`will become more apparent from the detailed description Set
`forth below when taken in conjunction with the drawings in
`which like reference characters identify correspondingly
`throughout and wherein:
`
`

`

`US 2004/0179627 A1
`
`Sep. 16, 2004
`
`FIG. 1 shows a multiple-access MIMO system;
`0016
`0017 FIG. 2 shows an exemplary frame structure for
`data transmission in a TDD MIMO-OFDM system;
`0.018
`FIG. 3 shows downlink and uplink pilot transmis
`Sions for an exemplary pilot transmission Scheme;
`0.019
`FIG. 4 shows a block diagram of an access point
`and a user terminal;
`0020 FIG. 5 shows a block diagram of a TX spatial
`processor that can generate a beacon pilot,
`0021
`FIG. 6A shows a block diagram of a TX spatial
`processor that can generate a MIMO pilot,
`0022 FIG. 6B shows a block diagram of an RX spatial
`processor that can provide a channel response estimate
`based on a received MIMO pilot;
`0023 FIG. 7A shows a block diagram of a TX spatial
`processor that can generate a steered reference; and
`0024 FIG. 7B shows a block diagram of an RX spatial
`processor that can provide a channel response estimate
`based on a received Steered reference.
`
`DETAILED DESCRIPTION
`0.025 The word “exemplary” is used herein to mean
`“Serving as an example, instance, or illustration.” Any
`embodiment or design described herein as “exemplary' is
`not necessarily to be construed as preferred or advantageous
`over other embodiments or designs.
`0026 FIG. 1 shows a multiple-access MIMO system 100
`that Supports a number of users and is capable of imple
`menting the pilots described herein. MIMO system 100
`includes a number of access points (APs) 110 that support
`communication for a number of user terminals (UTs) 120.
`For simplicity, only two access points 110a and 110b are
`shown in FIG. 1. An acceSS point is generally a fixed Station
`that is used for communicating with the user terminals. An
`access point may also be referred to as a base Station or using
`Some other terminology.
`0.027 User terminals 120 may be dispersed throughout
`the System. Each user terminal may be a fixed or mobile
`terminal that can communicate with the access point. A user
`terminal may also be referred to as an access terminal, a
`mobile station, a remote station, a user equipment (UE), a
`wireleSS device, or Some other terminology. Each user
`terminal may communicate with one or possibly multiple
`access points on the downlink and/or uplink at any given
`moment. The downlink (i.e., forward link) refers to trans
`mission from the access point to the user terminal, and the
`uplink (i.e., reverse link) refers to transmission from the user
`terminal to the access point. AS used herein, an “active' user
`terminal is one that is receiving a downlink transmission
`from an access point and/or transmitting an uplink trans
`mission to the access point.
`0028. In FIG. 1, access point 110a communicates with
`user terminals 120a through 120f, and access point 110b
`communicates with user terminals 120f through 120k. The
`assignment of user terminals to acceSS points is typically
`based on received Signal Strength and not distance. At any
`given moment, a user terminal may receive downlink trans
`mission from one or multiple access points. A System
`
`controller 130 couples to access points 110 and may be
`designed to perform a number of functions Such as (1)
`coordination and control for the access points coupled to it,
`(2) routing of data among these access points, and (3) access
`and control of communication with the user terminals Served
`by these access points.
`0029)
`I. Pilots
`0030 Pilots suitable for use in MIMO systems, such as
`the one shown in FIG. 1, are provided herein. These pilots
`can Support various functions that may be needed for proper
`System operation, Such as timing and frequency acquisition,
`channel estimation, calibration, and So on. The pilots may be
`considered as being of different types that are designed and
`used for different functions. Table 1 lists four types of pilot
`and their short description for an exemplary pilot design.
`Fewer, different, and/or additional pilot types may also be
`defined, and this is within the Scope of the invention.
`
`TABLE 1.
`
`Pilot Types
`
`Description
`Pilot Type
`Beacon Pilot A pilot transmitted from all transmit antennas and used for
`timing and frequency acquisition.
`MIMO Pilot A pilot transmitted from all transmit antennas with different
`orthogonal codes and used for channel estimation.
`Steered
`A pilot transmitted on specific eigenmodes of a MIMO
`Reference or channel for a specific user terminal and used for channel
`Steered Pilot estimation and possibly rate control.
`Carrier Pilot A pilot used for phase tracking of a carrier signal.
`
`0031 Steered reference and steered pilot are synonymous
`terms.
`0032 Various pilot transmission schemes may be devised
`based on any combination of these various types of pilot. For
`example, on the downlink, an access point may transmit a
`beacon pilot, a MIMO pilot, and a carrier pilot for all user
`terminals within its coverage area and may optionally trans
`mit a steered reference to any active user terminal that is
`receiving a downlink transmission from the acceSS point. On
`the uplink, a user terminal may transmit a MIMO pilot for
`calibration and may transmit a Steered reference and a carrier
`pilot when Scheduled (e.g., for downlink and/or uplink data
`transmissions). The processing to transmit and receive these
`various types of pilot is described in further detail below.
`0033. The pilots described herein may be used for various
`types of MIMO systems. For example, the pilots may be
`used for (1) single-carrier MIMO systems, (2) multi-carrier
`MIMO systems that employ orthogonal frequency division
`multiplexing (OFDM) or some other multi-carrier modula
`tion technique, (3) MIMO systems that implement multiple
`acceSS techniques Such as frequency division multiple-ac
`cess (FDMA), time division multiple-access (TDMA), and
`code division multiple-access (CDMA), (4) MIMO systems
`that implement frequency division multiplexing (FDM),
`time division multiplexing (TDM), and/or code division
`multiplexing (CDM) for data transmission, (5) MIMO sys
`tems that implement time division duplexing (TDD), fre
`quency division duplexing (FDD), and/or code division
`duplexing (CDD) for the downlink and uplink channels, and
`(6) other types of MIMO systems. For clarity, the pilots are
`
`

`

`US 2004/0179627 A1
`
`Sep. 16, 2004
`
`described below first for a MIMO system that implements
`OFDM (i.e., a MIMO-OFDM system) and then for a TDD
`MIMO-OFDM system.
`0034) OFDM effectively partitions the overall system
`bandwidth into a number of (N) orthogonal Subbands,
`which are also referred to as tones, frequency bins, or
`frequency subchannels. With OFDM, each Subband is asso
`ciated with a respective Subcarrier upon which data may be
`modulated. For a MIMO-OFDM system, each Subband may
`be associated with a number of eigenmodes, and each
`eigenmode of each Subband may be viewed as an indepen
`dent transmission channel.
`0035. For clarity, a specific pilot structure is described
`below for an exemplary MIMO-OFDM system. In this
`MIMO-OFDM system, the system bandwidth is partitioned
`into 64 orthogonal Subbands (i.e., N=64), which are
`assigned indices of -32 to +31. Of these 64 Subbands, 48
`Subbands (e.g., with indices of t1, . . . , 6, 8, ..., 20, 22,
`. . . , 26) may be used for data transmission, 4 Subbands
`(e.g., with indices of it 7, 21) may be used for a carrier
`pilot and possibly signaling, the DC Subband (with index of
`O) is not used, and the remaining Subbands are also not used
`and Serve as guard Subbands. Thus, of the 64 total Subbands,
`the 52 “usable” Subbands include the 48 data Subbands and
`4 pilot Subbands, and the remaining 12 Subbands are not
`used. This OFDM Subband structure is described in further
`detail in the aforementioned provisional U.S. Patent Appli
`cation Serial No. 60/421,309. Different number of Subbands
`and other OFDM Subband structures may also be imple
`mented for the MIMO-OFDM system, and this is within the
`Scope of the invention.
`0036). For OFDM, the data to be transmitted on each
`usable Subband is first modulated (i.e., Symbol mapped)
`using a particular modulation scheme (e.g., BPSK, QPSK,
`or M-QAM) selected for use for that Subband. One modu
`lation symbol may be transmitted on each usable Subband in
`each Symbol period. Each modulation Symbol is a complex
`value for a specific point in a signal constellation corre
`sponding to the Selected modulation Scheme. Signal values
`of Zero may be sent on the unused Subbands. For each
`OFDM symbol period, the modulation symbols for the
`uSable Subbands and Zero Signal values for the unused
`Subbands (i.e., the modulation Symbols and Zeros for all NE
`Subbands) are transformed to the time domain using an
`inverse fast Fourier transform (IFFT) to obtain a trans
`formed Symbol that comprises N time-domain Samples. To
`combat inter-symbol interference (ISI), a portion of each
`transformed symbol is often repeated (which is also referred
`to as adding a cyclic prefix) to form a corresponding OFDM
`symbol, which is then transmitted over the wireless channel.
`An OFDM symbol period, which is also referred to herein
`as a Symbol period, corresponds to the duration of one
`OFDM symbol.
`0037) 1. Beacon Pilot
`0.038. The beacon pilot includes a specific set of pilot
`symbols that is transmitted from each of the N transmit
`antennas. The Same Set of pilot Symbols is transmitted for
`N. symbol periods designated for beacon pilot transmission.
`In general, N may be any integer value of one or greater.
`0039. In an exemplary embodiment, the set of pilot
`symbols for the beacon pilot is a set of 12 BPSK modulation
`
`symbols for 12 specific Subbands, which is referred to as a
`“B” OFDM symbol. The 12 BPSK modulation symbols for
`the BOFDM symbol are given in Table 2. Signal values of
`ZeroS are transmitted on the remaining 52 unused Subbands.
`
`TABLE 2
`
`Pilot Symbols
`
`Beacon
`Plot
`b(k)
`
`MIMO
`Plot
`p(k)
`
`Subband
`Index
`
`-26
`-25
`-24
`-23
`-22
`-21
`-20
`-19
`-18
`-17
`-16
`-15
`-14
`-13
`-12
`-11
`-10
`-9
`-8
`-7
`-6
`-5
`-4
`-3
`-2
`-1
`O
`1.
`2
`3
`4
`5
`6
`7
`8
`9
`1O
`11
`12
`13
`14
`15
`16
`17
`18
`19
`2O
`21
`22
`23
`24
`25
`26
`
`O
`O
`O
`+ i
`O
`O
`O
`-1 - i
`O
`O
`O
`+ i
`O
`O
`O
`-1 - i
`O
`O
`O
`-1 - i
`O
`O
`O
`--
`O
`O
`O
`O
`O
`O
`O
`
`O
`O
`O
`
`O
`O
`O
`--
`O
`O
`O
`--
`O
`O
`O
`--
`O
`O
`O
`--
`O
`O
`O
`
`O
`-1 - i
`-1 + i
`-1 + i
`-1 + i
`- i.
`- i.
`+ i
`-1 - i
`-1 + i
`+ i
`-1 + i
`- i.
`+ i
`- i.
`- i.
`-1 - i
`-1 - i
`- i.
`-1 - i
`+ i
`-1 + i
`-1 - i
`-1 + i
`-1 + i
`- i.
`-1 + i
`O
`- i.
`-1 - i
`-1 - i
`-1 - i
`-1 + i
`+ i
`-1 - i
`-1 + i
`-1 - i
`-1 - i
`+ i
`- i.
`-1 + i
`-1 - i
`+ i
`-1 + i
`-1 + i
`- i.
`+ i
`-1 + i
`+ i
`-1 + i
`+ i
`-1 + i
`- i.
`-1 - i
`O
`
`0040 For the exemplary embodiment and as shown in
`Table 2, for the beacon pilot, the BPSK modulation symbol
`(1+) is transmitted in Subbands -24, -16, -4, 12, 16, 20, and
`24, and the BPSK modulation symbol -(1+j) is transmitted
`in Subbands -20, -12, -8, 4, and 8. Zero Signal values are
`transmitted on the remaining 52 Subbands for the beacon
`pilot.
`
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`US 2004/0179627 A1
`
`Sep. 16, 2004
`
`0041) The B OFDM symbol is designed to facilitate
`System timing and frequency acquisition by the user termi
`nals. For the exemplary embodiment of the B OFDM
`symbol described above, only 12 of the 64 total Subbands are
`used, and these Subbands are Spaced apart by four Subbands.
`This 4-Subband spacing allows the user terminal to have an
`initial frequency error of up to two Subbands. The beacon
`pilot allows the user terminal to correct for its initial coarse
`frequency error, and to correct its frequency So that the phase
`drift over the duration of the beacon pilot is Small (e.g., less
`than 45 degrees over the beacon pilot duration at a Sample
`rate of 20 MHz). If the beacon pilot duration is 8 usec, then
`the 45 degrees (or less) of phase drift over 8 usec is equal to
`360 degrees over 64 usec, which is approximately 16 kHz.
`0042. The 16 kHz frequency error is typically too large
`for operation. Additional frequency correction may be
`obtained using the MIMO pilot and the carrier pilot. These
`pilots span a long enough time duration that the user
`terminal frequency can be corrected to within the desired
`target (e.g., 250 Hz). For example, if a TDD frame is 2 msec
`(as described below) and if the user terminal frequency is
`accurate to within 250 Hz, then there will be less than half
`a cycle of phase change over one TDD frame. The phase
`difference from TDD frame to TDD frame of the beacon
`pilot may be used to lock the frequency of the user terminal
`to the clock at the acceSS point, thereby effectively reducing
`the frequency error to Zero.
`0043. In general, the set of pilot symbols used for the
`beacon pilot may be derived using any modulation Scheme.
`Thus, other OFDM symbols derived using BPSK or some
`other modulation Scheme may also be used for the beacon
`pilot, and this is within the Scope of the invention.
`0044) In an exemplary design, four transmit antennas are
`available for beacon pilot transmission. Table 4 lists the
`OFDM symbols to be transmitted from each of the four
`transmit antennas for a beacon pilot transmission that spans
`two Symbol periods.
`
`TABLE 3
`
`Beacon Pilot
`
`Symbol
`Period
`
`1.
`2
`
`Antenna 1
`
`Antenna 2
`
`Antenna 3
`
`Antenna 4
`
`B
`B
`
`B
`B
`
`B
`B
`
`B
`B
`
`0045 2. MIMO Pilot
`0046) The MIMO pilot includes a specific set of pilot
`symbols that is transmitted from each of the N transmit
`antennas. For each transmit antenna, the same Set of pilot
`Symbols is transmitted for NP. Symbol periods designated for
`MIMO pilot transmission. However, the set of pilot symbols
`for each transmit antenna is “covered” with a unique
`orthogonal Sequence or code assigned to that antenna. Cov
`ering is a process whereby a given pilot or data Symbol (or
`a set of L pilot/data symbols with the same value) to be
`transmitted is multiplied by all L chips of an L-chip orthogo
`nal Sequence to obtain L covered Symbols, which are then
`transmitted. Decovering is a complementary proceSS
`whereby received symbols are multiplied by the L chips of
`the same L-chip orthogonal Sequence to obtain L decovered
`
`Symbols, which are then accumulated to obtain an estimate
`of the transmitted pilot or data symbol. The covering
`achieves orthogonality among the N pilot transmissions
`from the N transmit antennas and allows a receiver to
`distinguish the individual transmit antennas, as described
`below. The duration of the MIMO pilot transmission may be
`dependent on its use, as described below. In general, NP may
`be any integer value of one or greater.
`0047 One set or different sets of pilot symbols may be
`used for the NT transmit antennas. In an exemplary embodi
`ment, one Set of pilot Symbols is used for all NT transmit
`antennas for the MIMO pilot and this set includes 52 QPSK
`modulation symbols for the 52 usable Subbands, which is
`referred to as a “P” OFDM symbol. The 52 QPSK modu
`lation symbols for the POFDM symbol are given in Table
`2. Signal values of Zero are transmitted on the remaining 12
`unused Subbands.
`0048. The 52 QPSK modulation symbols form a unique
`“word” that is designed to facilitate channel estimation by
`the user terminals. This unique word is Selected to have a
`minimum peak-to-average variation in a waveform gener
`ated based on these 52 modulation symbols.
`0049. It is well known that OFDM is generally associated
`with higher peak-to-average variation in the transmitted
`waveform than for Some other modulation technique (e.g.,
`CDMA). As a result, to avoid clipping of circuitry (e.g.,
`power amplifier) in the transmit chain, OFDM symbols are
`typically transmitted at a reduced power level, i.e., backed
`off from the peak transmit power level. The back-off is used
`to account for variations in the waveform for these OFDM
`Symbols. By minimizing the peak-to-average variation in the
`waveform for the POFDM symbol, the MIMO pilot may be
`transmitted at a higher power level (i.e., a Smaller back-off
`may be applied for the MIMO pilot). The higher transmit
`power for the MIMO pilot would then result in improved
`received signal quality for the MIMO pilot at the receiver.
`The Smaller peak-to-average variation may also reduce the
`amount of distortion and non-linearity generated by the
`circuitry in the transmit and receive chains. These various
`factors may result in improved accuracy for a channel
`estimate obtained based on the MIMO pilot.
`0050. An OFDM symbol with minimum peak-to-average
`variation may be obtained in various manners. For example,
`a random Search may be performed in which a large number
`of Sets of pilot Symbols are randomly formed and evaluated
`to find the Set that has the minimum peak-to-average varia
`tion. The POFDM symbol shown in Table 2 represents an
`exemplary OFDM symbol that may be used for the MIMO
`pilot. In general, the set of pilot symbols used for the MIMO
`pilot may be derived using any modulation Scheme. Thus,
`various other OFDM symbols derived using QPSK or some
`other modulation scheme may also be used for the MIMO
`pilot, and this is within the Scope of the invention.
`0051
`Various orthogonal codes may be used to cover the
`P OFDM symbols sent on the N transmit antennas.
`Examples of Such orthogonal codes include Walsh codes and
`orthogonal variable spreading factor (OVSF) codes. Pseudo
`orthogonal codes and quasi-orthogonal codes may also be
`used to cover the P OFDM symbols. An example of a
`pseudo-orthogonal code is the M-Sequence that is well
`known in the art. An example of a quasi-orthogonal code is
`the quasi-orthogonal function (QOF) defined by IS-2000. In
`
`

`

`US 2004/0179627 A1
`
`Sep. 16, 2004
`
`general, various types of codes may be used for covering,
`Some of which are noted above. For simplicity, the term
`“orthogonal code' is used herein to generically refer to any
`type of code Suitable for use for covering pilot symbols. The
`length (L) of the orthogonal code is selected to be greater
`than or equal to the number of transmit antennas (e.g.,
`LeN), and L orthogonal codes are available for use. Each
`transmit antenna is assigned a unique orthogonal code. The
`N POFDM symbols to be sent in N symbol periods from
`each transmit antenna are covered with the orthogonal code
`assigned to that transmit antenna.
`0.052
`In an exemplary embodiment, four transmit anten
`nas are available and are assigned 4-chip Walsh Sequences of
`W=1111, W=1010, W=1100, and W=1001 for the
`MIMO pilot. For a given Walsh sequence, a value of “1”
`indicates that a POFDM symbol is transmitted and a value
`of “0” indicates that a -P OFDM symbol is transmitted. For
`a -P OFDM symbol, each of the 52 QPSK modulation
`symbols in the POFDM symbol is inverted (i.e., multiplied
`with -1). The result of the covering for each transmit
`antenna is a sequence of covered POFDM symbols for that
`transmit antenna. The covering is in effect performed Sepa
`rately for each of the Subbands to generate a Sequence of
`covered pilot symbols for that Subband. The sequences of
`covered pilot symbols for all Subbands form the sequence of
`covered POFDM symbols.
`0053 Table 4 lists the OFDM symbols to be transmitted
`from each of the four transmit antennas for a MIMO pilot
`transmission that spans four Symbol periods.
`
`TABLE 4
`
`MIMO Pilot
`
`Symbol
`Period
`
`1.
`2
`3
`4
`
`Antenna 1
`
`Antenna 2
`
`Antenna 3
`
`Antenna 4
`
`--P
`--P
`--P
`--P
`
`--P
`-P
`--P
`-P
`
`--P
`--P
`-P
`-P
`
`--P
`-P
`-P
`--P
`
`0054 For this set of 4-chip Walsh sequences, the MIMO
`pilot transmission can occur in an integer multiple of four
`Symbol periods to ensure orthogonality among the four pilot
`transmissions from the four transmit antennas. The Walsh
`Sequence is simply repeated for a MIMO pilot transmission
`that is longer than the length of the Walsh Sequence.
`0055. The wireless channel for the MIMO-OFDM sys
`tem may be characterized by a set of channel response
`matrices H(k), for Subband index keK, where K=t{1 ... 26
`for the exemplary Subband structure described above. The
`matrix H(k) for each Subband includes NNR values, {h;
`i(k)}, for ie{1 . . . N} and je{1 . . . N}, where h;(k)
`represents the channel gain between the j-th transmit
`antenna and the i-th receive antenna.
`0056. The MIMO pilot may be used by the receiver to
`estimate the response of the wireleSS channel. In particular,
`to recover the pilot Sent from transmit antennai and received
`by receive antenna i, the received OFDM