US007095709B2
`
`(12) United States Patent
`US 7,095,709 B2
`(10) Patent N0.:
`
`Walton et al.
`(45) Date of Patent:
`Aug. 22, 2006
`
`(54) DIVERSITY TRANSMISSION MODES FOR
`MIMO OFDM COMMUNICATION SYSTEMS
`
`(75)
`
`Inventors: Jay R. Walton, Carlisle, MA (US);
`JOhn W‘ KetChum’ Harvard’ MA (Us)
`(73) Assignee: QUALCOMM, Incorporated, San
`Diego, CA (US)
`
`2002/0122381 A1*
`2002/0122383 A1*
`2002/0131516 A1*
`
`................... 370/208
`9/2002 Wu et al.
`................... 370/210
`9/2002 Wu et al.
`9/2002 El-Gamal et al.
`..
`...... 375/285
`
`...........
`011 e a .
`fir/:11? t~~~1~~~~~~~~~~~~~~~ gigggg
`2388;
`iggiégfigégg :1:
`
`.............. 370/208
`7/2003 Walton et al.
`2003/0128658 A1*
`2/2006 Jalall et al.
`2006/0023666 A1*
`................. 370/334
`FOREIGN PATENT DOCUMENTS
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 266 days.
`
`EP
`$8
`
`2/2002
`1 182 799 A2
`WO3003ZSS572 A2 * igggg;
`
`21
`
`)
`(
`(22)
`
`(65)
`
`A l. N .: 10/179 439
`pp
`0
`3
`Filed:
`Jun. 24, 2002
`
`Prior Publication Data
`
`* cited by examiner
`Primary ExamineriDuc Ho
`Assistant ExamineriPhuongchau Ba Nguyen
`(74) Attorney, Agent, or FirmiPhilip R. Wadsworth;
`Sandip (Micky) S. Minhas; Dmitry R- Milikevsky
`
`US 2003/0235147 A1
`
`Dec. 25, 2003
`
`(57)
`
`ABSTRACT
`
`(51)
`
`Int. Cl.
`(2006.01)
`H04J 11/00
`(52) US. Cl.
`....................... 370/208; 370/341; 375/267
`(58) Field of Classification Search ................ 370/210,
`370/208, 3293 209a 341a 342a 242a 5225 335;
`375/130S 285, 14647, 2673 299; 455/562,
`455/561, 67.11
`See application file for complete search history.
`_
`References Clted
`US. PATENT DOCUMENTS
`
`(56)
`
`5,511,067 A *
`4/1996 M11161“- ........................ 370/335
`
`6,067,290 A :
`----- 370/329
`5/2000 P311131] et 31~
`2’222’22; 31* 13388; gadglghkettalal “““““4357/2/72?
`,
`,
`.
`..
`1
`I111
`e
`.
`.
`
`2/2004 Kim et al. ............ 375/146
`6,690,712 B1*
`
`..... 370/208
`6,952,454 B1* 10/2005 Jalali et al.
`
`4/2002 Ma et al. .............. 375/267
`2002/0041635 A1*
`
`.............. 455/562
`
`8/2002 Hwang et a1.
`
`2002/0115473 A1*
`
`Techniques for transmitting data using a number of diversity
`transmission modes to improve reliability. At a transmitter,
`for each of one or more data streams, a particular diversity
`transmission mode is selected for use from among a number
`of possible transmission modes. These transmission modes
`may include a frequency diversity transmission mode, a
`Walsh diversity transmission mode, a space time transmit
`diversity (STTD) “31151111551011 mode, and a Walsh-STTD
`transmission mode. Each diversity transmission mode
`redundantly transmits data over time, frequency, space, or a
`combination thereof. Each data stream is coded and modu-
`lated to provide modulation symbols, which are further
`processed based on the selected diversity transmission mode
`to provide transmit symbols. For OFDM, the transmit sym-
`bols for all data streams are further OFDM modulated to
`~
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`1”?de a “if“? ngrafsmlsspe SymbOIS for web transmlt
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`3 Claims, 11 Drawing Sheets
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`Modulation
`Symbols
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`Transmit
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`Prefix
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`Generator
`
`Modulation
`Control
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`Transmission
`Mode Control
`
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`U.S. Patent
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`Aug. 22, 2006
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`Sheet 1 of 11
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`US 7,095,709 B2
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`U.S. Patent
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`Aug. 22, 2006
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`U.S. Patent
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`Aug. 22, 2006
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`U.S. Patent
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`Aug. 22, 2006
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`U.S. Patent
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`Aug. 22, 2006
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`U.S. Patent
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`Aug. 22, 2006
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`U.S. Patent
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`Aug. 22, 2006
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`U.S. Patent
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`U.S. Patent
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`Aug. 22, 2006
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`Sheet 10 0f 11
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`US 7,095,709 B2
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`U.S. Patent
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`Aug. 22, 2006
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`Sheet 11 0f 11
`
`US 7,095,709 B2
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`US 7,095,709 B2
`
`1
`DIVERSITY TRANSMISSION MODES FOR
`MIMO OFDM COMMUNICATION SYSTEMS
`
`BACKGROUND
`
`1. Field
`
`10
`
`25
`
`The present invention relates generally to data commu-
`nication, and more specifically to techniques for transmitting
`data using a number of diversity transmission modes in
`MlMO OFDM systems.
`2. Background
`Wireless communication systems are widely deployed to
`provide various types of communication such as voice,
`packet data, and so on. These systems may be multiple-
`access systems capable of supporting communication with 15
`multiple users either sequentially or simultaneously. This is
`achieved by sharing the available system resources, which
`may be quantified by the total available operating bandwidth
`and transmit power.
`Amultiple-access system may include a number of access 20
`points (or base stations) that communicate with a number of
`user terminals. Each access point may be equipped with one
`or multiple antennas for transmitting and receiving data.
`Similarly, each terminal may be equipped with one or
`multiple antennas.
`The transmission between a given access point and a
`given terminal may be characterized by the number of
`antennas used for data transmission and reception. In par-
`ticular, the access point and terminal pair may be viewed as
`(l) a multiple-input multiple-output (MlMO) system if 30
`multiple (NT) transmit antennas and multiple (NR) receive
`antennas are employed for data transmission, (2) a multiple-
`input single-output (MlSO) system if multiple transmit
`antennas and a single receive antenna are employed, (3) a
`single-input multiple-output
`(SIMO) system if a single 35
`transmit
`antenna
`and multiple receive
`antennas
`are
`employed, or (4) a single-input single-output (8180) system
`if a single transmit antenna and a single receive antenna are
`employed.
`For a MIMO system, a MIMO channel formed by the NT 40
`transmit and NR receive antennas may be decomposed into
`NS independent channels, with Nsémin {Np NR}. Each of
`the NS independent channels is also referred to as a spatial
`subchannel of the MIMO channel and corresponds to a
`dimension. The MIMO system can provide improved per- 45
`formance (e.g.,
`increased transmission capacity and/or
`greater reliability) if the additional dimensionalities created
`by the multiple transmit and receive antennas are utilized.
`For a MlSO system, only one spatial subchannel is available
`for data transmission. However, the multiple transmit anten- 50
`nas may be used to transmit data in a manner to improve the
`likelihood of correct reception by the receiver.
`The spatial subchannels of a wideband system may
`encounter different channel conditions due to various factors
`
`such as fading and multipath. Each spatial subchannel may 55
`thus experience frequency selective fading, which is char-
`acterized by different channel gains at different frequencies
`of the overall system bandwidth.
`It
`is well known that
`frequency selective fading causes inter-symbol interference
`(181), which is a phenomenon whereby each symbol in a 60
`received signal acts as distortion to subsequent symbols in
`the received signal. The 181 distortion degrades performance
`by impacting the ability to correctly detect the received
`symbols.
`To combat frequency selective fading, orthogonal fre-
`quency division multiplexing (OFDM) may be used to
`effectively partition the overall system bandwidth into a
`
`65
`
`2
`
`number of (NF) subbands, which may also be referred to as
`OFDM subbands, frequency bins, or frequency sub-chan-
`nels. Each subband is associated with a respective subcarrier
`upon which data may be modulated. For each time interval
`that may be dependent on the bandwidth of one subband, a
`modulation symbol may be transmitted on each of the NF
`subbands.
`
`For a multiple-access system, a given access point may
`communicate with terminals having different number of
`antennas at different times. Moreover, the characteristics of
`the communication channels between the access point and
`the terminals typically vary from terminal to terminal and
`may further vary over time, especially for mobile terminals.
`Different transmission schemes may then be needed for
`different
`terminals depending on their capabilities and
`requirements.
`There is therefore a need in the art for techniques for
`transmitting data using a number of diversity transmission
`modes depending on the capability of the receiver device
`and the channel conditions.
`
`SUMMARY
`
`Techniques are provided herein for transmitting data in a
`manner to improve the reliability of data transmission. A
`MlMO OFDM system may be designed to support a number
`of modes of operation for data transmission. These trans-
`mission modes may include diversity transmission modes,
`which may be used to achieve higher reliability for certain
`data transmission (e.g., for overhead channels, poor channel
`conditions, and so on). The diversity transmission modes
`attempt
`to achieve transmit diversity by establishing
`orthogonality among multiple signals transmitted from mul-
`tiple transmit antennas. Orthogonality among the transmit-
`ted signals may be attained in frequency, time, space, or any
`combination thereof. The transmission modes may also
`include spatial multiplexing transmission modes and beam
`steering transmission modes, which may be used to achieve
`higher bit rates under certain favorable channel conditions.
`In an embodiment, a method is provided for processing
`data for transmission in a wireless (e.g., MlMO OFDM)
`communication system. In accordance with the method, a
`particular diversity transmission mode to use for each of one
`or more data streams is selected from among a number of
`possible transmission modes. Each diversity transmission
`mode redundantly transmits data over time,
`frequency,
`space, or a combination thereof. Each data stream is coded
`and modulated based on coding and modulation schemes
`selected for the data stream to provide modulation symbols.
`The modulation symbols for each data stream are further
`processed based on the selected diversity transmission mode
`to provide transmit symbols. For OFDM, the transmit sym-
`bols for all data streams are further OFDM modulated to
`
`provide a stream of transmission symbols for each of one or
`more transmit antennas used for data transmission. Pilot
`
`symbols may also be multiplexed with the modulation
`symbols using frequency division multiplexing (FDM), time
`division multiplexing (TDM), code division multiplexing
`(CDM), or any combination thereof.
`The transmission modes may include, for example, (1) a
`frequency diversity transmission mode that redundantly
`transmits modulation symbols over multiple OFDM sub-
`bands, (2) a Walsh diversity transmission mode that trans-
`mits each modulation symbol over NT OFDM symbol peri-
`ods, where NT is the number of transmit antennas used for
`data transmission,
`(3) a space time transmit diversity
`(STTD) transmission mode that transmits modulation sym-
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`US 7,095,709 B2
`
`3
`bols over multiple OFDM symbol periods and multiple
`transmit antennas, and (4) a Walsh-STTD transmission
`mode that transmits modulation symbols using a combina-
`tion of Walsh diversity and STTD. For the Walsh diversity
`and Walsh-STTD transmission modes, the same modulation
`symbols may be redundantly transmitted over all transmit
`antennas or different modulation symbols may be transmit-
`ted over different transmit antennas.
`
`Each data stream may be for an overhead channel or
`targeted for a specific receiver device. The data rate for each
`user-specific data stream may be adjusted based on the
`transmission capability of the receiver device. The transmit
`symbols for each data stream are transmitted on a respective
`group of one or more subbands.
`In another embodiment, a method is provided for pro-
`cessing a data transmission at a receiver of a wireless
`communication system. In accordance with the method, the
`particular diversity transmission mode used for each of one
`or more data streams to be recovered is initially determined.
`The diversity transmission mode used for each is selected
`from among a number of possible transmission modes.
`Received symbols for each data stream are then processed
`based on the diversity transmission mode used for the data
`stream to provide recovered symbols, which are estimates of
`modulation symbols transmitted from a transmitter for the
`data stream. The recovered symbols for each data stream are
`further demodulated and decoded to provide decoded data
`for the data stream.
`
`10
`
`15
`
`20
`
`25
`
`Various aspects and embodiments of the invention are
`described in further detail below. The invention further
`
`30
`
`provides methods, transmitter units, receiver units, termi-
`nals, access points, systems, and other apparatuses and
`elements that implement various aspects, embodiments, and
`features of the invention, as described in further detail
`below.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The features, nature, and advantages of the present inven-
`tion will become more apparent from the detailed descrip-
`tion set forth below when taken in conjunction with the
`drawings in which like reference characters identify corre-
`spondingly throughout and wherein:
`FIG. 1 is a diagram of a multiple-access system that
`supports a number of users;
`FIG. 2 is a block diagram of an embodiment of an access
`point and two terminals;
`FIG. 3 is a block diagram of a transmitter unit;
`FIG. 4 is a block diagram of a TX diversity processor that
`may be used to implement the frequency diversity scheme;
`FIG. 5 is a block diagram of a TX diversity processor that
`may be used to implement the Walsh diversity scheme;
`FIG. 6 is a block diagram of a TX diversity processor that
`may be used to implement the STTD scheme;
`FIG. 7 is a block diagram of a TX diversity processor that
`may be used to implement a repeated Walsh-STTD scheme;
`FIG. 8 is a block diagram of a TX diversity processor that
`may be used to implement a non-repeated Walsh-STTD
`scheme;
`FIG. 9 is a block diagram of a receiver unit;
`FIG. 10 is a block diagram of an RX diversity processor;
`FIG. 11 is a block diagram of an RX antenna processor
`within the RX diversity processor and which may be used
`for the Walsh diversity scheme; and
`FIG. 12 is a block diagram of an RX subband processor
`within the RX antenna processor and which may be used for
`the repeated and non-repeated Walsh-STTD schemes.
`
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`DETAILED DESCRIPTION
`
`FIG. 1 is a diagram of a multiple-access system 100 that
`supports a number of users. System 100 includes one or
`more access points (AP) 104 that communicate with a
`number of terminals (T) 106 (only one access point is shown
`in FIG. 1 for simplicity). An access point may also be
`referred to as a base station, a UTRAN, or some other
`terminology. Aterminal may also be referred to as a handset,
`a mobile station, a remote station, user equipment (UE), or
`some other terminology. Each terminal 106 may concur-
`rently communicate with multiple access points 104 when in
`soft handoif (if soft handolf is supported by the system).
`In an embodiment, each access point 104 employs mul-
`tiple antennas and represents (1) the multiple-input (MI) for
`a downlink transmission from the access point to a terminal
`and (2) the multiple-output (MO) for an uplink transmission
`from the terminal to the access point. A set of one or more
`terminals 106 communicating with a given access point
`collectively represents the multiple-output for the downlink
`transmission and the multiple-input for the uplink transmis-
`sion.
`
`Each access point 104 can communicate with one or
`multiple terminals 106, either concurrently or sequentially,
`via the multiple antennas available at the access point and
`the one or more antennas available at each terminal. Termi-
`
`nals not in active communication may receive pilots and/or
`other signaling information from the access point, as shown
`by the dashed lines for terminals 1066 through 106k in FIG.
`1.
`
`For the downlink, the access point employs NT antennas
`and each terminal employs l or NR antennas for reception of
`one or more data streams from the access point. In general,
`NR can be different for different multi-antenna terminals and
`can be any integer. A MIMO channel formed by the NT
`transmit antennas and NR receive antennas may be decom-
`posed into NS independent channels, with Nsémin {Np
`NR}. Each such independent channel is also referred to as a
`spatial subchannel of the MIMO channel. The terminals
`concurrently receiving downlink data transmission need not
`be equipped with equal number of receive antennas.
`For the downlink, the number of receive antennas at a
`given terminal may be equal to or greater than the number
`of transmit antennas at the access point (i.e., NRENT). For
`such a terminal, the number of spatial subchannels is limited
`by the number of transmit antennas at the access point. Each
`multi-antenna terminal communicates with the access point
`via a respective MIMO channel formed by the access point’s
`NT transmit antennas and its own NR receive antennas.
`However, even if multiple multi-antenna terminals are
`selected for concurrent downlink data transmission, only NS
`spatial subchannels are available regardless of the number of
`terminals receiving the downlink transmission.
`For the downlink, the number of receive antennas at a
`given terminal may also be less than the number of transmit
`antennas at the access point (i.e., NR<NT). For example, a
`MISO terminal is equipped with a single receive antenna
`(NR:l) for downlink data transmission. The access point
`may then employ diversity, beam steering, space division
`multiple access (SDMA), or some other transmission tech-
`niques to communicate simultaneously with one or multiple
`MISO terminals.
`
`For the uplink, each terminal may employ a single
`antenna or multiple antennas for uplink data transmission.
`Each terminal may also utilize all or only a subset of its
`available antennas for uplink transmission. At any given
`moment, the NT transmit antennas for the uplink are formed
`
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`5
`by all antennas used by one or more active terminals. The
`MIMO channel is then formed by the NT transmit antennas
`from all active terminals and the access point’s NR receive
`antennas. The number of spatial subchannels is limited by
`the number of transmit antennas, which is typically limited
`by the number of receive antennas at the access point (i.e.,
`Nsémin {Np NR}).
`FIG. 2 is a block diagram of an embodiment of access
`point 104 and two terminals 106. On the downlink, at access
`point 104, various types of traffic data such as user-specific
`data from a data source 208, signaling, and so on are
`provided to a transmit (TX) data processor 210. Processor
`210 then formats and encodes the traffic data based on one
`
`or more coding schemes to provide coded data. The coded
`data is then interleaved and further modulated (i.e., symbol
`mapped) based on one or more modulation schemes to
`provide modulation symbols (i.e., modulated data). The data
`rate, coding,
`interleaving, and symbol mapping may be
`determined by controls provided by a controller 230 and a
`scheduler 234. The processing by TX data processor 210 is
`described in further detail below.
`
`A transmit processor 220 then receives and processes the
`modulation symbols and pilot data to provide transmission
`symbols. The pilot data is typically known data processed in
`a known manner, if at all. In a specific embodiment, the
`processing by transmit processor 220 includes (1) process-
`ing the modulation symbols based on one or more transmis-
`sion modes selected for use for data transmission to the
`
`terminals to provide transmit symbols and (2) OFDM pro-
`cessing the transmit symbols to provide transmission sym-
`bols. The processing by transmit processor 220 is described
`in further detail below.
`
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`Transmit processor 220 provides NT streams of transmis-
`sion symbols to NT transmitters (TMTR) 222a through 2221,
`one transmitter for each antenna used for data transmission.
`
`35
`
`Each transmitter 222 converts its transmission symbol
`stream into one or more analog signals and further condi-
`tions (e.g., amplifies, filters, and frequency upconverts) the
`analog signals to generate a respective downlink modulated
`signal suitable for transmission over a wireless communi-
`cation channel. Each downlink modulated signal
`is then
`transmitted via a respective antenna 224 to the terminals.
`At each terminal 106, the downlink modulated signals
`from multiple transmit antennas of the access point are
`received by one or multiple antennas 252 available at the
`terminal. The received signal from each antenna 252 is
`provided to a respective receiver (RCVR) 254. Each receiver
`254 conditions (e.g., filters, amplifies, and frequency down-
`converts) its received signal and further digitizes the con-
`ditioned signal to provide a respective stream of samples.
`A receive processor 260 then receives and processes the
`streams of samples from all receivers 254 to provide recov-
`ered symbols (i.e., demodulated data). In a specific embodi-
`ment, the processing by receive processor 260 includes (1)
`OFDM processing the received transmission symbols to
`provide received symbols, and (2) processing the received
`symbols based on the selected transmission mode(s) to
`obtain recovered symbols. The recovered symbols are esti-
`mates of the modulation symbols transmitted by the access
`point. The processing by receive processor 260 is described
`in further detail below.
`
`A receive (RX) data processor 262 then symbol demaps,
`deinterleaves, and decodes the recovered symbols to obtain
`the user-specific data and signaling transmitted on the down-
`link for the terminal. The processing by receive processor
`260 and RX data processor 262 is complementary to that
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`performed by transmit processor 220 and TX data processor
`210, respectively, at the access point.
`On the uplink, at terminal 106, various types of traffic data
`such as user-specific data from a data source 276, signaling,
`and so on are provided to a TX data processor 278. Processor
`278 codes the different types of traffic data in accordance
`with their respective coding schemes to provide coded data
`and further interleaves the coded data. A modulator 280 then
`
`symbol maps the interleaved data to provide modulated data,
`which is provided to one or more transmitters 254. OFDM
`may or may not be used for the uplink data transmission,
`depending on the system design. Each transmitter 254
`conditions the received modulated data to generate a respec-
`tive uplink modulated signal, which is then transmitted via
`an associated antenna 252 to the access point.
`At access point 104, the uplink modulated signals from
`one or more terminals are received by antennas 224. The
`received signal from each antenna 224 is provided to a
`receiver 222, which conditions and digitizes the received
`signal to provide a respective stream of samples. The sample
`streams from all receivers 222 are then processed by a
`demodulator 240 and further decoded (if necessary) by an
`RX data processor 242 to recover the data transmitted by the
`terminals.
`
`Controllers 230 and 270 direct the operation at the access
`point and the terminal, respectively. Memories 232 and 272
`provide storage for program codes and data used by con-
`trollers 230 and 270, respectively. Scheduler 234 schedules
`the data transmission on the downlink (and possibly the
`uplink) for the terminals.
`For clarity, various transmit diversity schemes are spe-
`cifically described below for downlink transmission. These
`schemes may also be used for uplink transmission, and this
`is within the scope of the invention. Also for clarity, in the
`following description, subscript “i” is used as an index for
`the receive antennas, subscript “j” is used as an index for the
`transmit antennas, and subscript “k” is used as an index for
`the subbands in the MIMO OFDM system.
`
`Transmitter Unit
`
`FIG. 3 is a block diagram of a transmitter unit 300, which
`is an embodiment of the transmitter portion of access point
`104. Transmitter unit 300 includes (1) a TX data processor
`210a that receives and processes traffic and pilot data to
`provide modulation symbols and (2) a transmit processor
`220a that further processes the modulation symbols to
`provide NT streams of transmission symbols for the NT
`transmit antennas. TX data processor 210a and transmit
`processor 220a are one embodiment of TX data processor
`210 and transmit processor 220, respectively, in FIG. 2.
`In the specific embodiment shown in FIG. 3; TX data
`processor 210a includes an encoder 312, a channel inter-
`leaver 314, and a symbol mapping element 316. Encoder
`312 receives and codes the traffic data (i.e., the information
`bits) based on one or more coding schemes to provide coded
`bits. The coding increases the reliability of the data trans-
`mission.
`
`In an embodiment, the user-specific data for each terminal
`and the data for each overhead channel may be considered
`as distinct data streams. The overhead channels may include
`broadcast, paging, and other common channels intended to
`be received by all terminals. Multiple data streams may also
`be sent to a given terminal. Each data stream may be coded
`independently based on a specific coding scheme selected
`for that data stream. Thus, a number of independently coded
`
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`7
`data streams may be provided by encoder 312 for different
`overhead channels and terminals.
`
`The specific coding scheme to be used for each data
`stream is determined by a coding control from controller
`230. The coding scheme for each terminal may be selected,
`for example, based on feedback information received from
`the terminal. Each coding scheme may include any combi-
`nation of forward error detection (FED) codes (e. g., a cyclic
`redundancy check (CRC) code) and forward error correction
`(FEC) codes (e.g., a convolutional code, a Turbo code, a
`block code, and so on). A coding scheme may also designate
`no coding at all. Binary or trellis-based codes may also be
`used for each data stream. Moreover, with convolutional and
`Turbo codes, puncturing may be used to adjust the code rate.
`More specifically, puncturing may be used to increase the
`code rate above the base code rate.
`
`In a specific embodiment, the data for each data stream is
`initially partitioned into frames (or packets). For each frame,
`the data may be used to generate a set of CRC bits for the
`frame, which is then appended to the data. The data and CRC
`bits for each frame are then coded with either a convolu-
`
`tional code or a Turbo code to generate the coded data for the
`frame.
`Channel
`
`interleaver 314 receives and interleaves the
`
`coded bits based on one or more interleaving schemes.
`Typically, each coding scheme is associated with a corre-
`sponding interleaving scheme. In this case, each indepen-
`dently coded data stream would be interleaved separately.
`The interleaving provides time diversity for the coded bits,
`permits each data stream to be transmitted based on an
`average SNR of the subbands and spatial subchannels used
`for the data stream, combats fading, and further removes
`correlation between coded bits used to form each modula-
`
`tion symbol.
`With OFDM, the channel interleaver may be designed to
`distribute the coded data for each data stream over multiple
`subbands of a single OFDM symbol or possibly over mul-
`tiple OFDM symbols. The objective of the channel inter-
`leaver is to randomize the coded data so that the likelihood
`
`of consecutive coded bits being corrupted by the commu-
`nication channel is reduced. When the interleaving interval
`for a given data stream spans a single OFDM symbol, the
`coded bits for the data stream are randomly distributed
`across the subbands used for the data stream to exploit
`frequency diversity. When the interleaving interval spans
`multiple OFDM symbols,
`the coded bits are randomly
`distributed across the data-carrying subbands and the multi-
`symbol interleaving interval to exploit both frequency and
`time diversity. For a wireless local area network (WLAN),
`the time diversity realized by interleaving over multiple
`OFDM symbols may not be significant if the minimum
`expected coherence time of the communication channel is
`many times longer than the interleaving interval.
`Symbol mapping element 316 receives and maps the
`interleaved data in accordance with one or more modulation
`
`schemes to provide modulation symbols. A particular modu-
`lation scheme may be used for each data stream. The symbol
`mapping for each data stream may be achieved by grouping
`sets of qm coded and interleaved bits to form data symbols
`(each of which may be a non-binary value), and mapping
`each data symbol to a point in a signal constellation corre-
`sponding to the modulation scheme selected for use for that
`data stream. The selected modulation scheme may be QPSK,
`M-PSK, M-QAM, or some other modulation scheme. Each
`mapped signal point is a complex value and corresponds to
`an Mm-ary modulation symbol, where Mm corresponds to
`the specific modulation scheme selected for data stream m
`
`8
`and Mm:2q“. Symbo