(12) United States Patent
`Wallace et al.
`
`US006473467B1
`US 6,473,467 B1
`Oct. 29, 2002
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`(54) METHOD AND APPARATUS FOR
`MEASURING REPORTING CHANNEL STATE
`
`6,141,393 A * 10/2000 Thomas et a1. ........... .. 375/347
`6,144,711 A * 11/2000 Raleigh et a1. . . . . .
`. . . .. 375/347
`
`INFORMATION IN A HIGH EFFICIENCY’
`
`SYSTEM
`
`. . . .. 370/208
`6,151,296 A * 11/2000 Vijayan et a1. . . . . .
`6,151,328 A * 11/2000 KWOII CI 8.1. .............. .. 370/441
`
`FOREIGN PATENT DOCUMENTS
`
`(75) Inventors: Mark Wallace, Bedford, MA (US); Jay
`R. Walton, Westford, MA (US);
`Ahmad Jalali, San Diego, CA (US)
`
`EP
`WO
`
`0 683 576 A1 * 11/1995
`00/04728
`1/2000
`OTHER PUBLICATIONS
`
`(73) Assignee: Qualcomm Incorporated, San Diego,
`CA (US)
`
`( * ) NOIiCeI
`
`Subject I0 any disclaimer, the term of this
`patent is eXIended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`K.L. Baum et al., “A Comparison of Differential and Coher
`ent Reception for a Coded OFDM System in a LoW C/I
`Environment,” IEEE Global Telecommunications Confer
`ence. Phoenix, Arizona, Nov. 3—8, 1987, Global Telecom
`munications Conference NeW York, IEEE, Us, vol. 1, Nov.
`3, 1997 (pp. 300—304).
`
`(21) Appl. N0.: 09/539,224
`.
`_
`(22) Flled'
`
`Mar‘ 30’ 2000
`Related US. Application Data
`
`(63) Continuation-in-part of application No. 09/532,492, ?led 0n
`Maf- 22, 2000-
`(51) Int. Cl.7 ........................... .. H04B 7/02; H04] 11/00
`(52) U S C]
`375/267_ 375/144_ 375060
`'
`'
`' """"
`375/347’_ 370005; 370016
`F, M f S
`h
`’
`375/144 148
`1e
`0
`370/208’ 210?
`4 5’5 /1 01’
`
`’
`
`’
`
`’
`
`’
`
`’
`
`58
`(
`)
`
`*
`
`.
`
`.
`
`‘med by exammer
`Primary Examiner—Young T. Tse
`(74) Attorney, Agent, or Firm—Philip R. Wadsworth; Kent
`D‘ Baker; Kyong Macek
`57
`ABSTRACT
`(
`)
`Channel state information (CSI) can be used by a commu
`nicatioPs systsm to PreCQHditiOQ transmissions between
`transmitter units and receiver units. In one aspect of the
`invention, disjoint sub-channel sets are assigned to transmit
`antennas located at a transmitter unit. Pilot symbols are
`generated and transmitted on a subset of the disjoint sub
`channels. Upon receipt of the transmitted pilot symbols, the
`receiver units determine the CS1 for the disjoint sub
`channels that carried pilot symbols. These CSI values are
`reported to the transmitter unit, Which Will use these CSI
`values to generate CSI estimates for the disjoint sub
`channels that did not carry pilot symbols. The amount of
`information necessary to report CS1 on the reverse link can
`.
`.
`.
`.
`.
`be further ImmFmZed thrf’ugh Compresslon techmques and
`resource allocatlon teehnlqlles
`
`29 Claims, 12 Drawing Sheets
`
`(56)
`
`References Cited
`
`US. PATENT DOCUMENTS
`
`5,170,413 A * 12/1992 Hess et a1. ................ .. 375/260
`
`5274 836 A * 12 1993
`
`5:748:683 A * 5/1998 Smith et a1. .............. .. 375/347
`5,790,516 A * 8/1998 Gudmundson et a1.
`370/210
`5,914,933 A * 6/1999 Cimini et a1. ............. .. 370/208
`5,933,421 A * 8/1999 Alamouti et a1. ......... .. 370/330
`
`..... .. 455 1
`
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`PRECONDl'llUN DATA; AND
`ASSIGN PRECONDlTIONED DATA
`TO MULTIPLE SUB'CHANNELS
`AND ANTENNAS
`
`14 If
`
`PERFORM IFFT oN DATA
`ASSIGNED 'io
`EACH ANTENNA
`
`'i RANSMIT FROM
`MULTIPLE ANTENNAS
`
`RECEIVE AT
`MULTIPLE ANTENNAS
`
`PERFORM FFT
`
`MEASURE CSl 0N PLLO'I: AND
`DEMODUIATT‘. USING (‘SI
`
`149
`
`CSI
`FEEDBACK
`
`Page 1 of 29
`
`

`
`U.S. Patent
`
`Oct. 29, 2002
`
`Sheet 1 of 12
`
`US 6,473,467 B1
`
`2:
`
`«J.05
`
`Page 2 of 29
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`Page 2 of 29
`
`

`
`U.S. Patent
`
`0a. 29, 2002
`
`Sheet 2 0f 12
`
`US 6,473,467 B1
`
`PRECONDITION DATA; AND
`ASSIGN PRECONDITIONED DATA
`TO MULTIPLE SUB-CHANNELS
`AND ANTENNAS
`
`141)
`
`V
`
`(142
`
`PERFORM IFFT ON DATA
`ASSIGNED TO
`EACH ANTENNA
`
`[143
`‘V
`APPEND CYCLIC
`PREFIX/EXTENSION
`
`[144
`‘V
`TRANSMIT FROM
`MULTIPLE ANTENNAS
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`(147
`PERFORM FFT
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`148
`/
`v
`MEASURE CSI ON PILOT; AND
`DEMODULATE USING C81
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`Page 3 of 29
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`

`
`U.S. Patent
`
`0a. 29, 2002
`
`Sheet 3 0f 12
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`US 6,473,467 B1
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`Page 4 of 29
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`

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`U.S. Patent
`
`Oct. 29, 2002
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`US 6,473,467 B1
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`

`
`US 6,473,467 B1
`
`1
`METHOD AND APPARATUS FOR
`MEASURING REPORTING CHANNEL STATE
`INFORMATION IN A HIGH EFFICIENCY,
`HIGH PERFORMANCE COMMUNICATIONS
`SYSTEM
`
`CROSS REFERENCE TO RELATED
`APPLICATIONS
`
`This is a continuation-in-part of co-pending application
`Ser. No. 09/532,492, entitled “HIGH EFFICIENCY, HIGH
`PERFORMANCE COMMUNICATIONS SYSTEM
`EMPLOYING MULTI-CARRIER MODULATION,” ?led
`on Mar. 22, 2000, assigned to the assignee of the present
`invention, and incorporated herein by reference.
`
`10
`
`15
`
`BACKGROUND OF THE INVENTION
`
`I. Field of the Invention
`The present invention relates to the ?eld of communica
`tions. More particularly, the present invention relates to the
`measurement and report of channel state information in a
`high ef?ciency, high performance communications system.
`II. Description of the Related Art
`A modern day Wireless communications system is
`required to operate over channels that eXperience fading and
`multipath. One such communications system is a code
`division multiple access (CDMA) system that conforms to
`the “TIA/EIA/IS-95 Mobile Station-Base Station Compat
`ibility Standard for Dual-Mode Wideband Spread Spectrum
`Cellular System,” hereinafter referred to as the IS-95 stan
`dard. The CDMA system supports voice and data commu
`nication betWeen users over a terrestrial link. The use of
`CDMA techniques in a multiple access communication
`system is disclosed in US. Pat. No. 4,901,307, entitled
`“SPREAD SPECTRUM MULTIPLE ACCESS COMMU
`NICAT ION SYSTEM USING SATELLITE OR TERRES
`TRIAL REPEATERS,” and US. Pat. No. 5,103,459, entitled
`“SYSTEM AND METHOD FOR GENERATING WAVE
`FORMS IN A CDMA CELLULAR TELEPHONE
`SYSTEM,” both assigned to the assignee of the present
`invention and incorporated herein by reference.
`An IS-95 system can operate ef?ciently by estimating
`channel parameters at a receiver unit, Which uses these
`estimated channel parameters to demodulate a received
`signal. The IS-95 system makes channel estimation ef?cient
`by requiring the transmission of a pilot signal from every
`base station. This pilot signal is a repeating PN-type
`sequence knoWn by the receiver unit. Correlation of the
`received pilot signal With a local replica of the pilot signal
`enables the receiver unit to estimate the complex impulse
`response of the channel and adjust demodulator parameters
`accordingly. For the IS-95 Waveform and system parameters
`it is not necessary or bene?cial to report information on the
`channel conditions measured by the receiver unit back to the
`transmitter unit.
`Given the ever-groWing demand for Wireless
`communication, a higher ef?ciency, higher performance
`Wireless communications system is desirable. One type of
`higher performance Wireless communications system is a
`Multiple Input/Multiple Output (MIMO) system that
`employs multiple transmit antennas to transmit over a propa
`gation channel to multiple receive antennas. As in loWer
`performance systems, the propagation channel in a MIMO
`system is subject to the deleterious effects of multipath, as
`Well as interference from adjacent antennas. Multipath
`occurs When a transmitted signal arrives at a receiver unit
`
`20
`
`25
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`30
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`35
`
`40
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`45
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`55
`
`60
`
`65
`
`2
`through multiple propagation paths With differing delays.
`When signals arrive from multiple propagation paths, com
`ponents of the signals can combine destructively, Which is
`referred to as “fading.” In order to improve the ef?ciency
`and decrease the complexity of the MIMO system, infor
`mation as to the characteristics of the propagation channel
`can be transmitted back to the transmitter unit in order to
`precondition the signal before transmission.
`Preconditioning the signal can be dif?cult When the
`characteristics of the propagation channel change rapidly.
`The channel response can change With time due to the
`movement of the receiver unit or changes in the environment
`surrounding the receiver unit. Given a mobile environment,
`an optimal performance requires that information regarding
`channel characteristics, such as fading and interference
`statistics, be determined and transmitted quickly to the
`transmitter unit before the channel characteristics change
`signi?cantly. As delay of the measurement and reporting
`process increases, the utility of the channel response infor
`mation decreases. A present need eXists for efficient tech
`niques that Will provide rapid determination of the channel
`characteristics.
`
`SUMMARY OF THE INVENTION
`
`The present invention is directed to a method and appa
`ratus for the measuring and reporting of channel state
`information in a high ef?ciency, high performance commu
`nications system, comprising the steps of: generating a
`plurality of pilot signals; transmitting the plurality of pilot
`signals over a propagation channel betWeen a transmitter
`unit and a plurality of receiver units, Wherein the transmitter
`unit comprises at least one transmit antenna, each of the
`plurality of receiver units comprises at least one receive
`antenna, and the propagation channel comprises a plurality
`of sub-channels betWeen the transmitter unit and the plural
`ity of receiver units; receiving at least one of the plurality of
`pilot signals at each of the plurality of receiver units;
`determining a set of transmission characteristics for at least
`one of the plurality of sub-channels, Wherein the step of
`determining the set of transmission characteristics uses at
`least one of the plurality of pilot signals received at each of
`the plurality of receiver units; reporting an information
`signal from each of the plurality of receiver units to the
`transmitter unit, Wherein the information signal carries the
`set of transmission characteristics for at least one of the
`plurality of sub-channels; and optimiZing a set of transmis
`sion parameters at the transmitter unit, based on the infor
`mation signal.
`In one aspect of the invention, pilot symbols are trans
`mitted on a plurality of disjoint OFDM sub-channel sets.
`When the pilot symbols are transmitted on disjoint OFDM
`sub-channels, the characteristics of the propagation channel
`can be determined through a set of K sub-channels carrying
`the pilot symbols, Wherein K is less than the number of
`OFDM sub-channels in the system. In addition to transmit
`ting pilot symbols on disjoint sub-channels, the system can
`transmit a time-domain pilot sequence that can be used to
`determine characteristics of the propagation channel. Along
`With the generation and transmission of pilot symbols, an
`aspect of the invention is the compression of the amount of
`information necessary to reconstruct the characteristics of
`the propagation channel.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The features, nature, and advantages of the present inven
`tion Will become more apparent from the detailed descrip
`
`Page 14 of 29
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`

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`US 6,473,467 B1
`
`3
`tion set forth below When taken in conjunction With the
`drawings in Which like reference characters identify corre
`spondingly throughout and Wherein:
`FIG. 1A is a diagram of a multiple-input multiple-output
`(MIMO) communications system;
`FIG. 1B is a diagram of a OFDM-based MIMO system
`With feedback of channel state information;
`FIG. 1C is a diagram of an exemplary OFDM pilot signal
`structure that can be used to estimate the channel state
`information;
`FIG. 2 is a diagram that graphically illustrates a speci?c
`example of a transmission from a transmit antenna at a
`transmitter unit;
`FIG. 3 is a block diagram of a data processor and a
`modulator of the communications system shoWn in FIG. 1A;
`FIGS. 4A and 4B are block diagrams of tWo versions of
`a channel data processor that can be used for processing one
`channel data stream such as control, broadcast, voice, or
`traf?c data;
`FIGS. 5A through 5C are block diagrams of the process
`ing units that can be used to generate the transmit signal
`shoWn in FIG. 2;
`FIG. 6 is a block diagram of a receiver unit, having
`multiple receive antennas, Which can be used to receive one
`or more channel data streams; and
`FIG. 7 shoWs plots that illustrate the spectral ef?ciency
`achievable With some of the operating modes of a commu
`nications system in accordance With one embodiment.
`
`DETAILED DESCRIPTION OF THE SPECIFIC
`EMBODIMENTS
`
`FIG. 1A is a diagram of a Multiple Input/Multiple Output
`(MIMO) communications system 100 capable of imple
`menting some embodiments of the invention. Communica
`tions system 100 can be operative to provide a combination
`of antenna, frequency, and temporal diversity to increase
`spectral efficiency, improve performance, and enhance ?ex
`ibility. Increased spectral efficiency is characteriZed by the
`ability to transmit more bits per second per HertZ (bps/HZ)
`When and Where possible to better utiliZe the available
`system bandWidth. Techniques to obtain higher spectral
`ef?ciency are described in further detail beloW. Improved
`performance may be quanti?ed, for example, by a loWer
`bit-error-rate (BER) or frame-error-rate (FER) for a given
`link carrier-to-noise-plus-interference ratio (C/I). And
`enhanced ?exibility is characteriZed by the ability to accom
`modate multiple users having different and typically dispar
`ate requirements. These goals may be achieved, in part, by
`employing multi-carrier modulation, time division multi
`plexing (TDM), multiple transmit and/or receive antennas,
`and other techniques. The features, aspects, and advantages
`of the invention are described in further detail beloW.
`As shoWn in FIG. 1A, communications system 100
`includes a ?rst system 110 in communication With a second
`system 120. System 110 includes a (transmit) data processor
`112 that (1) receives or generates data, (2) processes the data
`to provide antenna, frequency, or temporal diversity, or a
`combination thereof, and (3) provides processed modulation
`symbols to a number of modulators (MOD) 114a through
`114i. Each modulator 114 further processes the modulation
`symbols and generates an RF modulated signal suitable for
`transmission. The RF modulated signals from modulators
`114a through 114[ are then transmitted from respective
`antennas 116a through 116[ over communications links 118
`to system 120.
`
`10
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`
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`
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`
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`
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`In FIG. 1A, system 120 includes a number of receive
`antennas 122a through 122r that receive the transmitted
`signals and provide the received signals to respective
`demodulators (DEMOD) 124a through 124r. As shoWn in
`FIG. 1A, each receive antenna 122 may receive signals from
`one or more transmit antennas 116 depending on a number
`of factors such as, for example, the operating mode used at
`system 110, the directivity of the transmit and receive
`antennas, the characteristics of the communications links,
`and others. Each demodulator 124 demodulates the respec
`tive received signal using a demodulation scheme that is
`complementary to the modulation scheme used at the trans
`mitter. The demodulated symbols from demodulators 124a
`through 124r are then provided to a (receive) data processor
`126 that further processes the symbols to provide the output
`data. The data processing at the transmitter and receiver
`units is described in further detail beloW.
`FIG. 1A shoWs only the forWard link transmission from
`system 110 to system 120. This con?guration may be used
`for data broadcast and other one-Way data transmission
`applications. In a bidirectional communications system, a
`reverse link from system 120 to system 110 is also provided,
`although not shoWn in FIG. 1A for simplicity. For the
`bidirectional communications system, each of systems 110
`and 120 may operate as a transmitter unit or a receiver unit,
`or both concurrently, depending on Whether data is being
`transmitted from, or received at, the unit.
`For simplicity, communications system 100 is shoWn to
`include one transmitter unit (i.e., system 110) and one
`receiver unit (i.e., system 120). HoWever, in general, mul
`tiple transmit antennas and multiple receive antennas are
`present on each transmitter unit and each receiver unit. The
`communications system of the invention may include any
`number of transmitter units and receiver units.
`Each transmitter unit may include a single transmit
`antenna or a number of transmit antennas, such as that
`shoWn in FIG. 1A. Similarly, each receiver unit may include
`a single receive antenna or a number of receive antennas,
`again such as that shoWn in FIG. 1A. For example, the
`communications system may include a central system (i.e.,
`similar to a base station in the IS-95 CDMA system) having
`a number of antennas that transmit data to, and receive data
`from, a number of remote systems (i.e., subscriber units,
`similar to remote stations in the CDMA system), some of
`Which may include one antenna and others of Which may
`include multiple antennas.
`As used herein, an antenna refers to a collection of one or
`more antenna elements that are distributed in space. The
`antenna elements may be physically located at a single site
`or distributed over multiple sites. Antenna elements physi
`cally co-located at a single site may be operated as an
`antenna array (e.g., such as for a CDMA base station). An
`antenna netWork consists of a collection of antenna arrays or
`elements that are physically separated (e.g., several CDMA
`base stations). An antenna array or an antenna netWork may
`be designed With the ability to form beams and to transmit
`multiple beams from the antenna array or netWork. For
`example, a CDMA base station may be designed With the
`capability to transmit up to three beams to three different
`sections of a coverage area (or sectors) from the same
`antenna array. Thus, the three beams may be vieWed as three
`transmissions from three antennas.
`The communications system of the invention can be
`designed to provide a multi-user, multiple access commu
`nications scheme capable of supporting subscriber units
`having different requirements as Well as capabilities. The
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`5
`scheme allows the system’s total operating bandwidth, W,
`(e.g., 1.2288 MHZ) to be efficiently shared among different
`types of services that may have highly disparate data rate,
`delay, and quality of service (QOS) requirements.
`Examples of such disparate types of services include
`voice services and data services. Voice services are typically
`characteriZed by a loW data rate (e.g., 8 kbps to 32 kbps),
`short processing delay (e.g., 3 msec to 100 msec overall
`one-Way delay), and sustained use of a communications
`channel for an extended period of time. The short delay
`requirements imposed by voice services typically require a
`small fraction of the system resources to be dedicated to
`each voice call for the duration of the call. In contrast, data
`services are characteriZed by “bursty” traf?cs in Which
`variable amounts of data are sent at sporadic times. The
`amount of data can vary signi?cantly from burst-to-burst
`and from user-to-user. For high efficiency, the communica
`tions system of the invention can be designed With the
`capability to allocate a portion of the available resources to
`voice services as required and the remaining resources to
`data services. A fraction of the available system resources
`may also be dedicated for certain data services or certain
`types of data services.
`The distribution of data rates achievable by each sub
`scriber unit can vary Widely betWeen some minimum and
`maximum instantaneous values (e.g., from 200 kbps to over
`20 Mbps). The achievable data rate for a particular sub
`scriber unit at any given moment may be in?uenced by a
`number of factors such as the amount of available transmit
`poWer, the quality of the communications link (i.e., the C/I),
`the coding scheme, and others. The data rate requirement of
`each subscriber unit may also vary Widely betWeen a mini
`mum value (e.g., 8 kbps, for a voice call) all the Way up to
`the maximum supported instantaneous peak rate (e.g., 20
`Mbps for bursty data services).
`The percentage of voice and data traffic is typically a
`random variable that changes over time. In accordance With
`certain aspects of the invention, to efficiently support both
`types of services concurrently, the communications system
`of the invention is designed With the capability to dynamic
`allocate the available resources based on the amount of
`voice and data traffic. A scheme to dynamically allocate
`resources is described beloW. Another scheme to allocate
`resources is described in the aforementioned US. patent
`application Ser. No. 08/963,386.
`The communications system of the invention provides the
`above-described features and advantages, and is capable of
`supporting different types of services having disparate
`requirements. The features are achieved by employing
`antenna, frequency, or temporal diversity, or a combination
`thereof. Antenna, frequency, or temporal diversity can be
`independently achieved and dynamically selected.
`As used herein, antenna diversity refers to the transmis
`sion and/or reception of data over more than one antenna,
`frequency diversity refers to the transmission of data over
`more than one sub-band, and temporal diversity refers to the
`transmission of data over more than one time period.
`Antenna, frequency, and temporal diversity may include
`subcategories. For example, transmit diversity refers to the
`use of more than one transmit antenna in a manner to
`improve the reliability of the communications link, receive
`diversity refers to the use of more than one receive antenna
`in a manner to improve the reliability of the communications
`link, and spatial diversity refers to the use of multiple
`transmit and receive antennas to improve the reliability
`and/or increase the capacity of the communications link.
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`Transmit and receive diversity can also be used in combi
`nation to improve the reliability of the communications link
`Without increasing the link capacity. Various combinations
`of antenna, frequency, and temporal diversity can thus be
`achieved and are Within the scope of the present invention.
`Frequency diversity can be provided by use of a multi
`carrier modulation scheme such as orthogonal frequency
`division multiplexing (OFDM), Which alloWs for transmis
`sion of data over various sub-bands of the operating band
`Width. Temporal diversity is achieved by transmitting the
`data over different times, Which can be more easily accom
`plished With the use of time-division multiplexing (TDM).
`These various aspects of the communications system of the
`invention are described in further detail beloW.
`In accordance With an aspect of the invention, antenna
`diversity is achieved by employing a number of (NT) trans
`mit antennas at the transmitter unit or a number of (NR)
`receive antennas at the receiver unit, or multiple antennas at
`both the transmitter and receiver units. In a terrestrial
`communications system (e.g., a cellular system, a broadcast
`system, an MMDS system, and others), an RF modulated
`signal from a transmitter unit may reach the receiver unit via
`a number of transmission paths. The characteristics of the
`transmission paths typically vary over time based on a
`number of factors. If more than one transmit or receive
`antenna is used, and if the transmission paths betWeen the
`transmit and receive antennas are independent (i.e.,
`uncorrelated), Which is generally true to at least an extent,
`then the likelihood of correctly receiving the transmitted
`signal increases as the number of antennas increases.
`Generally, as the number of transmit and receive antennas
`increases, diversity increases and performance improves.
`Antenna diversity is dynamically provided based on the
`characteristics of the communications link to provide the
`required performance. For example, a higher degree of
`antenna diversity can be provided for some types of com
`munication (e.g., signaling), for some types of services (e.g.,
`voice), for some communications link characteristics (e.g.,
`loW C/I), or for some other conditions or considerations.
`As used herein, antenna diversity includes transmit diver
`sity and receive diversity. For transmit diversity, data is
`transmitted over multiple transmit antennas. Typically, addi
`tional processing is performed on the data transmitted from
`the transmit antennas to achieve the desired diversity. For
`example, the data transmitted from different transmit anten
`nas may be delayed or reordered in time, or coded and
`interleaved across the available transmit antennas. Also,
`frequency and temporal diversity may be used in conjunc
`tion With the different transmit antennas. For receive
`diversity, modulated signals are received on multiple receive
`antennas, and diversity is achieved by simply receiving the
`signals via different transmission paths.
`In accordance With another aspect of the invention, fre
`quency diversity can be achieved by employing a multi
`carrier modulation scheme. One such scheme that has
`numerous advantages is OFDM. With OFDM modulation,
`the overall transmission channel is essentially divided into a
`number of (L) parallel sub-channels that are used to transmit
`the same or different data. The overall transmission channel
`occupies the total operating bandWidth of W, and each of the
`sub-channels occupies a sub-band having a bandWidth of
`W/L and centered at a different center frequency. Each
`sub-channel has a bandWidth that is a portion of the total
`operating bandWidth. Each of the sub-channels may also be
`considered an independent data transmission channel that
`may be associated With a particular (and possibly unique)
`processing, coding, and modulation scheme, as described
`beloW.
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`7
`The data may be partitioned and transmitted over any
`de?ned set of tWo or more sub-bands to provide frequency
`diversity. For example, the transmission to a particular
`subscriber unit may occur over sub-channel 1 at time slot 1,
`sub-channel 5 at time slot 2, sub-channel 2 at time slot 3, and
`so on. As another example, data for a particular subscriber
`unit may be transmitted over sub-channels 1 and 2 at time
`slot 1 (e.g., With the same data being transmitted on both
`sub-channels), sub-channels 4 and 6 at time slot 2, only
`sub-channel 2 at time slot 3, and so on. Transmission of data
`over different sub-channels over time can improve the
`performance of a communications system experiencing fre
`quency selective fading and channel distortion. Other ben
`e?ts of OFDM modulation are described beloW.
`In accordance With yet another aspect of the invention,
`temporal diversity is achieved by transmitting data at dif
`ferent times, Which can be more easily accomplished using
`time division multiplexing (TDM). For data services (and
`possibly for voice services), data transmission occurs over
`time slots that may be selected to provide immunity to time
`dependent degradation in the communications link. Tempo
`ral diversity may also be achieved through the use of
`interleaving.
`For example, the transmission to a particular subscriber
`unit may occur over time slots 1 through x, or on a subset
`of the possible time slots from 1 through x (e.g., time slots
`1, 5, 8, and so on). The amount of data transmitted at each
`time slot may be variable or ?xed. Transmission over
`multiple time slots improves the likelihood of correct data
`reception due to, for example, impulse noise and interfer
`ence.
`The combination of antenna, frequency, and temporal
`diversity alloWs the communications system of the invention
`to provide robust performance. Antenna, frequency, and/or
`temporal diversity improves the likelihood of correct recep
`tion of at least some of the transmitted data, Which may then
`be used (e.g., through decoding) to correct for some errors
`that may have occurred in the other transmissions. The
`combination of antenna, frequency, and temporal diversity
`also alloWs the communications system to concurrently
`accommodate different types of services having disparate
`data rate, processing delay, and quality of service require
`ments.
`The communications system of the invention can be
`designed and operated in a number of different communi
`cations modes, With each communications mode employing
`antenna, frequency, or temporal diversity, or a combination
`thereof. The communications modes include, for example, a
`diversity communications mode and a MIMO communica
`tions mode. Various combinations of the diversity and
`MIMO communications modes can also be supported by the
`communications system. Also, other communications modes
`can be implemented and are Within the scope of the present
`invention.
`The diversity communications mode employs transmit
`and/or receive diversity, frequency, or temporal diversity, or
`a combination thereof, and is generally used to improve the
`reliability of the communications link. In one implementa
`tion of the diversity communications mode, the transmitter
`unit selects a modulation and coding scheme (i.e.,
`con?guration) from a ?nite set of possible con?gurations,
`Which are knoWn to the receiver units. For example, each
`overhead and common channel may be associated With a
`particular con?guration that is knoWn to all receiver units.
`When using the diversity communications mode for a spe
`ci?c user (e.g., for a voice call or a data transmission), the
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`mode and/or con?guration may be knoWn a priori (e.g., from
`a previous set up) or negotiated (e.g., via a common channel)
`by the receiver unit.
`In the diversity communications mode, data is transmitted
`on