(12) United States Patent
`Tong et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,120,395 B2
`Oct. 10, 2006
`
`US007120395 B2
`
`(54) MIMO COMMUNICATIONS
`(75) Inventors: Wen Tong, Ottawa (CA); Ming Jia,
`Ottawa (CA); Peiying Zhu, Kanata
`(CA)
`(73) Assignee: Nortel Networks Limited, St. Laurent
`(CA)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 372 days.
`(21) Appl. No.: 10/689,433
`
`(*) Notice:
`
`(22) Filed:
`
`Oct. 20, 2003
`
`(65)
`
`Prior Publication Data
`US 2005/0085.195 A1
`Apr. 21, 2005
`(51) Int. Cl.
`(2006.01)
`H04B 7/00
`(52) U.S. Cl. ........................ 455/101; 455/69; 455/561;
`455/562.1375/349
`(58) Field of Classification Search ............. 455/562.1,
`455/101, 70, 103; 370/334; 375/349, 299
`See application file for complete search history.
`References Cited
`U.S. PATENT DOCUMENTS
`
`(56)
`
`6,580,926 B1* 6/2003 Levy .......................... 455,564
`6,917,820 B1* 7/2005 Gore et al. .............. 455,562.1
`2003/0003880 A1* 1/2003 Ling et al. .................... 455/92
`2003.0161282 A1
`8/2003 Medvedev et al. ......... 370,329
`2004/0002364 A1
`1/2004 Trikkonen et al. ....... 455,562.1
`2004/0023621 A1* 2/2004 Sugar et al. ................ 455,103
`
`
`
`9, 2004 Haustein et al. ............ 455,450
`2004/0171385 A1
`2005.0003863 A1* 1/2005 Gorokhov .........
`... 455,562.1
`2005/0085269 A1 * 4/2005 Buljore et al. ........... 455,562.1
`FOREIGN PATENT DOCUMENTS
`
`EP
`EP
`WO
`WO
`
`O95.1091 A2 10, 1999
`1185.001 A2
`6, 2002
`O1 78254 A1 10, 2001
`O1/95531 A2 12/2001
`
`OTHER PUBLICATIONS
`International Search Report for PCT/GB2004/004443 mailed Jan.
`26, 2005.
`* cited by examiner
`Primary Examiner—George Eng
`Assistant Examiner Brandon J. Miller
`(74) Attorney, Agent, or Firm Withrow & Terranova,
`PLLC
`
`(57)
`ABSTRACT
`The present invention allows a wireless communication
`system, such as a base station, to select Nantennas from an
`associated group of M antennas for transmitting multiple
`streams of data to a given user. Based on the channel
`conditions between the Mantennas of the wireless commu
`nication system and the multiple antennas at the receiver, the
`Nantennas to use for transmission are selected to enhance
`channel capacity, signal-to-noise ratios, or a combination
`thereof. The channel conditions are measured at the receiver,
`and may be sent back to the wireless communication system
`for processing or may be processed at the receiver, wherein
`instructions are transmitted back to the wireless communi
`cation system to control antenna selection.
`
`36 Claims, 9 Drawing Sheets
`
`1
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`GM 1005
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`

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`U.S. Patent
`U.S. Patent
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`Oct. 10, 2006
`Oct. 10, 2006
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`Sheet 1 of 9
`Sheet 1 of 9
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`US 7,120,395 B2
`US 7,120,395 B2
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`FIG. 1
`FIG. 1
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`2
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`U.S. Patent
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`, 2006
`Oct. 10
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`Sheet 2 of 9
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`US 7,120,395 B2
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`WELSÅS TIO!H_LNOO
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`

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`U.S. Patent
`U.S. Patent
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`Oct. 10, 2006
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`Sheet 3 of 9
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`US 7,120,395 B2
`US 7,120,395 B2
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`U.S. Patent
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`Oct. 10, 2006
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`U.S. Patent
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`US 7,120,395 B2
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`26
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`U.S. Patent
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`Oct. 10, 2006
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`Sheet 6 of 9
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`US 7,120,395 B2
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`MIMO
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`-an-b-
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`ENCODING D<
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`MMO
`DECODING
`
`FIG. 6A
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`MMO
`ENCODING
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`MIMO
`DECODNG
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`MIMO
`ENCODING
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`MIMO
`DECODING
`
`FIG. 6C
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`7
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`U.S. Patent
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`Oct. 10, 2006
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`Sheet 7 of 9
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`US 7,120,395 B2
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`MIMO
`ENCODING
`s(i)
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`MIMO
`DECODING
`
`MIMO
`ENCODING
`s (1)
`1
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`MIMO
`DECODING
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`
`
`MMO
`
`ENCODING >
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`MIMO
`DECODING
`
`FIG. 7C
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`8
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`U.S. Patent
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`Oct. 10, 2006
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`Sheet 8 of 9
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`US 7,120,395 B2
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`MIMO
`ENCODING
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`MIMO
`DECODING
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`FIG. 8A
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`MIMO
`ENCODING
`s (1)
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`MIMO
`DECODNG
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`FIG. 8B
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`9
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`Oct. 10, 2006
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`Sheet 9 Of 9
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`US 7,120,395 B2
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`MIMO
`DECODING
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`MIMO
`ENCODING
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`40
`1 N7
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`402
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`403
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`N/
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`N/
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`404
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`FIG. 8C
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`10
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`

`

`1.
`MIMO COMMUNICATIONS
`
`FIELD OF THE INVENTION
`
`US 7,120,395 B2
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`2
`SUMMARY OF THE INVENTION
`
`The present invention allows a wireless communication
`system, such as a base station, to select Nantennas from an
`associated group of M antennas for transmitting multiple
`streams of data to a given user. Based on the channel
`conditions between the Mantennas of the wireless commu
`nication system and the multiple antennas at the receiver, the
`Nantennas to use for transmission are selected to enhance
`channel capacity, signal-to-noise ratios, or a combination
`thereof. The channel conditions are measured at the receiver,
`and may be sent back to the wireless communication system
`for processing or may be processed at the receiver, wherein
`instructions are transmitted back to the wireless communi
`cation system to control antenna selection.
`In an alternative embodiment, one or more of the M
`antennas, other than the selected Nantennas, may be used to
`redundantly transmit corresponding data streams. The
`redundant data streams are weighted in a manner allowing
`the redundant data stream to reinforce a corresponding data
`stream transmitted from one of the N antennas in the
`transmission channel. Further, the primary data stream,
`which is to be reinforced, may also be weighted prior to
`transmission. The preferred space-time coding uses BLAST
`transmission techniques.
`Those skilled in the art will appreciate the scope of the
`present invention and realize additional aspects thereof after
`reading the following detailed description of the preferred
`embodiments in association with the accompanying drawing
`figures.
`
`BRIEF DESCRIPTION OF THE DRAWING
`FIGURES
`
`The accompanying drawing figures incorporated in and
`forming a part of this specification illustrate several aspects
`of the invention, and together with the description serve to
`explain the principles of the invention.
`FIG. 1 is a block representation of a wireless communi
`cation system according to one embodiment of the present
`invention.
`FIG. 2 is a block representation of a base station accord
`ing to one embodiment of the present invention.
`FIG. 3 is a block representation of a user element accord
`ing to one embodiment of the present invention.
`FIG. 4 is a logical breakdown of a transmitter architecture
`according to one embodiment of the present invention.
`FIG. 5 is a logical breakdown of a receiver architecture
`according to one embodiment of the present invention.
`FIGS. 6A through 6C illustrate exemplary transmission
`diversity arrangements according to a first embodiment of
`the present invention.
`FIGS. 7A through 7C illustrate exemplary beam-forming
`arrangements according to a second embodiment of the
`present invention.
`FIGS. 8A through 8C illustrate exemplary beam-forming
`arrangements according to a third embodiment of the present
`invention.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`The embodiments set forth below represent the necessary
`information to enable those skilled in the art to practice the
`invention and illustrate the best mode of practicing the
`invention. Upon reading the following description in light of
`the accompanying drawing figures, those skilled in the art
`
`The present invention relates to wireless communications,
`and in particular to selectively choosing a select number of
`antennas within a larger array through which to transmit data
`to a receiver, as well as techniques for reinforcing transmit
`ted signals using available antennas.
`
`5
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`10
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`BACKGROUND OF THE INVENTION
`
`15
`
`Spatial diversity is typically a function of the number and
`placement of transmit and receive antennas relative to a
`transmitter and receiver. Systems employing spatial diver
`sity with multiple transmit and receive antennas are gener
`ally referred to as multiple-input multiple-output (MIMO)
`systems. Accordingly, a transmitting device will have M
`transmit antennas, and the receiving device will have N
`receive antennas. A common MIMO transmission technique
`transmits different data from different transmit antennas.
`This transmission technique, which is often referred to as
`V-BLAST (Vertical-Bell Laboratories Layered Space Time),
`increases system throughput for systems having Sufficient
`25
`diversity available. Further information related to BLAST
`techniques can be found in Siavash M. Alamouti, “A Simple
`Transmit Diversity Technique for Wireless Communica
`tions.” IEEE J. Select. Areas Commun., vol. 16, pp.
`1451–1458, Oct. 1998; G. J. Foschini, “Layered Space-time
`Architecture for Wireless Communications in a Fading
`Environment when Using Multi-element antennas.” Bell
`Labs Tech. J., pp. 41-59, Autumn 1996; G. D. Golden, G. J.
`Foschini, R. A. Valenzuela, and P. W. Wolniansky, “Detec
`tion Algorithm and Initial Laboratory Results. Using
`V-BLAST Space-time Communication Architecture.” Elec
`tronics Letters, vol. 35, pp. 14–16, Jan. 1999; and P. W.
`Wolniansky, G. J., Foschini, G. D. Golden, and R. A.
`Valenzuela, “V-BLAST: An Architecture for Realizing Very
`High Data Rates Over the Rich-scattering Wireless Chan
`40
`nel.” Proc. IEEE ISSSE-98, Pisa, Italy, Sep. 1998, pp.
`295-300, which are incorporated herein by reference.
`Interference and fading are significant impediments to
`achieving high data rates in today's wireless communication
`systems and is particularly problematic in MIMO systems.
`Given the tendency for channel conditions to sporadically
`and significantly fade, communication resources are conser
`Vatively allocated, leaving excessive amounts of communi
`cation resources unused most of the time.
`In cellular embodiments, the transmitter at a base station
`will typically be associated with more antennas than are
`used to receive transmitted signals at the receiver of a user
`element. As the user element moves or environmental con
`ditions change, the channel conditions between any one of
`the transmit antennas and the receive antennas may signifi
`cantly change. As such, at any given moment there are
`communication channels between the transmit and receive
`antennas that are better than others. In an effort to capitalize
`on those channels providing better transmission conditions,
`there is a need for a technique to facilitate transmissions
`from the transmitter to the receiver in a manner taking
`advantage of these more favorable channel conditions. Fur
`ther, there is a need to provide signal reinforcement in the
`communication channel to provide a beam-forming effect in
`an efficient manner in a MIMO system, and in particular in
`a MIMO system incorporating BLAST encoding and decod
`1ng.
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`3
`will understand the concepts of the invention and will
`recognize applications of these concepts not particularly
`addressed herein. It should be understood that these con
`cepts and applications fall within the scope of the disclosure
`and the accompanying claims.
`The present invention optimizes multiple-input multiple
`output (MIMO) transmissions wherein there are more trans
`mit antennas than receive antennas. In operation, channel
`conditions associated with each of the transmission paths
`from the M transmit antennas to the N receive antennas are
`either known or provided to the transmitter. Based on the
`channel information, the MXN MIMO system is reduced to
`an NXN MIMO subsystem, wherein only N of the M
`transmit antennas are used for transmitting data to the
`receiver. In one embodiment, the unselected transmit anten
`15
`nas may be used for reinforcing transmissions from the N
`transmit antennas. Prior to delving into the details of the
`invention, an overview of a MIMO environment is provided.
`With reference to FIG. 1, a basic MIMO wireless com
`munication environment is illustrated. In general, a base
`station controller (BSC) 10 controls wireless communica
`tions within one or more cells 12, which are served by
`corresponding base stations (BS) 14. Each base station 14
`facilitates communications with user elements 16, which are
`within the cell 12 associated with the corresponding base
`station 14. For the present invention, the base stations 14 and
`user elements 16 include multiple antennas to provide
`spatial diversity for communications. Notably, the base
`station 14 may be any type of wireless access point for
`cellular, wireless local area network, or like wireless net
`work.
`With reference to FIG. 2, a base station 14 configured
`according to one embodiment of the present invention is
`illustrated. The base station 14 generally includes a control
`system 20, a baseband processor 22, transmit circuitry 24,
`receive circuitry 26, multiple antennas 28, and a network
`interface 30. The receive circuitry 26 receives radio fre
`quency signals through antennas 28 bearing information
`from one or more remote transmitters provided by user
`elements 16. Preferably, a low noise amplifier and a filter
`(not shown) cooperate to amplify and remove broadband
`interference from the signal for processing. Downconver
`sion and digitization circuitry (not shown) will then down
`convert the filtered, received signal to an intermediate or
`baseband frequency signal, which is then digitized into one
`or more digital streams.
`The baseband processor 22 processes the digitized
`received signal to extract the information or data bits con
`veyed in the received signal. This processing typically
`comprises demodulation, decoding, and error correction
`operations. As such, the baseband processor 22 is generally
`implemented in one or more digital signal processors
`(DSPs). The received information is then sent across a
`wireless network via the network interface 30 or transmitted
`to another user element 16 serviced by the base station 14.
`The network interface 30 will typically interact with the base
`station controller 10 and a circuit-switched network forming
`a part of a wireless network, which may be coupled to the
`public switched telephone network (PSTN).
`On the transmit side, the baseband processor 22 receives
`digitized data, which may represent voice, data, or control
`information, from the network interface 30 under the control
`of the control system 20, and encodes the data for transmis
`Sion. The encoded data is output to the transmit circuitry 24,
`where it is modulated by a carrier signal having a desired
`transmit frequency or frequencies. A power amplifier (not
`shown) will amplify the modulated carrier signal to a level
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`appropriate for transmission, and deliver the modulated
`carrier signal to the antennas 28 through a matching network
`(not shown). The multiple antennas 28 and the replicated
`transmit and receive circuitries 24, 26 provide spatial diver
`sity. Antenna selection, modulation, and processing details
`are described in greater detail below.
`With reference to FIG. 3, a user element 16 configured
`according to one embodiment of the present invention is
`illustrated. Similarly to the base station 14, the user element
`16 will include a control system 32, a baseband processor
`34, transmit circuitry 36, receive circuitry 38, multiple
`antennas 40, and user interface circuitry 42. The receive
`circuitry 38 receives radio frequency signals through anten
`nas 40 bearing information from one or more base stations
`14. Preferably, a low noise amplifier and a filter (not shown)
`cooperate to amplify and remove broadband interference
`from the signal for processing. Downconversion and digi
`tization circuitry (not shown) will then downconvert the
`filtered, received signal to an intermediate or baseband
`frequency signal, which is then digitized into one or more
`digital streams.
`The baseband processor 34 processes the digitized
`received signal to extract the information or data bits con
`veyed in the received signal. This processing typically
`comprises demodulation, decoding, and error correction
`operations, as will be discussed on greater detail below. The
`baseband processor 34 is generally implemented in one or
`more digital signal processors (DSPs) and application spe
`cific integrated circuits (ASICs).
`For transmission, the baseband processor 34 receives
`digitized data, which may represent voice, data, or control
`information, from the control system 32, which it encodes
`for transmission. The encoded data is output to the transmit
`circuitry 36, where it is used by a modulator to modulate a
`carrier signal that is at a desired transmit frequency or
`frequencies. A power amplifier (not shown) will amplify the
`modulated carrier signal to a level appropriate for transmis
`Sion, and deliver the modulated carrier signal to the antennas
`40 through a matching network (not shown). The multiple
`antennas 40 and the replicated transmit and receive circuit
`ries 36, 38 provide spatial diversity. Modulation and pro
`cessing details are described in greater detail below.
`With reference to FIG. 4, a logical transmission architec
`ture is provided according to one embodiment. The trans
`mission architecture is described as being that of the base
`station 14, but those skilled in the art will recognize the
`applicability of the illustrated architecture for both uplink
`and downlink communications. Further, the transmission
`architecture is intended to represent a variety of multiple
`access architectures, including, but not limited to code
`division multiple access (CDMA), frequency division mul
`tiple access (FDMA), time division multiple access
`(TDMA), and orthogonal frequency division multiplexing
`(OFDM).
`Initially, the base station controller 10 sends data 44
`intended for a user element 16 to the base station 14 for
`scheduling. The scheduled data 44, which is a stream of bits,
`is scrambled in a manner reducing the peak-to-average
`power ratio associated with the data using data scrambling
`logic 46. A cyclic redundancy check (CRC) for the
`scrambled data is determined and appended to the scrambled
`data using CRC adding logic 48. Next, channel coding is
`performed using channel encoder logic 50 to effectively add
`redundancy to the data to facilitate recovery and error
`correction at the user element 16. The channel encoder logic
`50 uses known Turbo encoding techniques in one embodi
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`5
`ment. The encoded data is then processed by rate matching
`logic 52 to compensate for the data expansion associated
`with encoding.
`Bit interleaver logic 54 systematically reorders the bits in
`the encoded data to minimize the loss of consecutive data
`bits. The resultant data bits are systematically mapped into
`corresponding symbols depending on the chosen baseband
`modulation by mapping logic 56. Preferably, a form of
`Quadrature Amplitude Modulation (QAM) or Quadrature
`Phase Shift Key (QPSK) modulation is used. The symbols
`may be systematically reordered to further bolster the immu
`nity of the transmitted signal to periodic data loss caused by
`frequency selective fading using symbol interleaver logic
`58.
`At this point, groups of bits have been mapped into
`symbols representing locations in an amplitude and phase
`constellation. Blocks of symbols are then processed by
`space-time code (STC) encoder logic 60. The STC encoder
`logic 60 will process the incoming symbols according to a
`selected STC encoding mode and provide in outputs corre
`sponding to the number of transmit antennas 28 for the base
`station 14. Further detail regarding the STC encoding is
`provided later in the description. Assume the symbols for the
`in outputs are representative of the data to be transmitted and
`capable of being recovered by the user element 16. Further
`detail is provided in A. F. Naguib, N. Seshadri, and A. R.
`Calderbank, 'Applications of space-time codes and inter
`ference Suppression for high capacity and high data rate
`wireless systems.” Thirty-Second Asilomar Conference on
`Signals, Systems & Computers, Volume 2, pp. 1803–1810,
`1998; R. van Nee, A. Van Zelst and G. A. Atwater, “Maxi
`mum Likelihood Decoding in a Space Division Multiplex
`System”, IEEE VTC. 2000, pp. 6–10, Tokyo, Japan, May
`2000; and P. W. Wolniansky et al., “V-BLAST: An Archi
`tecture for Realizing Very High Data Rates over the Rich
`Scattering Wireless Channel.” Proc. IEEE ISSSE-98, Pisa,
`Italy, Sep. 30, 1998 which are incorporated herein by
`reference in their entireties.
`For illustration, assume the base station 14 has selected
`two of a number of antennas 28 (n=2) and the STC encoder
`logic 60 provides two output streams of symbols. Accord
`ingly, each of the symbol streams output by the STC encoder
`logic 60 is sent to a corresponding multiple access modu
`lation function 62, illustrated separately for ease of under
`standing. Those skilled in the art will recognize that one or
`more processors may be used to provide Such analog or
`digital signal processing alone or in combination with other
`processing described herein. For example, the multiple
`access modulation function 62 in a CDMA function would
`provide the requisite PN code multiplication, wherein an
`OFDM function would operate on the respective symbols
`using inverse discrete Fourier transform (IDFT) or like
`processing to effect an Inverse Fourier Transform. Attention
`is drawn to co-assigned application Ser. No. 10/104.399,
`filed Mar. 22, 2002, entitled SOFT HANDOFF FOR OFDM,
`for additional OFDM details, and to RF Microelectronics by
`Behzad Razavi, 1998 for CDMA and other multiple access
`technologies, both of which are incorporated herein by
`reference in their entirety.
`Each of the resultant signals is up-converted in the digital
`domain to an intermediate frequency and converted to an
`analog signal via the corresponding digital up-conversion
`(DUC) circuitry 64 and digital-to-analog (D/A) conversion
`circuitry 66. The resultant analog signals are then simulta
`neously modulated at the desired RF frequency, amplified,
`and transmitted via the RF circuitry 68 and antennas 28.
`Notably, the transmitted data may be preceded by pilot
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`signals, which are known by the intended user element 16.
`The user element 16, which is discussed in detail below, may
`use the pilot signals for channel estimation and interference
`suppression and the header for identification of the base
`station 14.
`Reference is now made to FIG. 5 to illustrate reception of
`the transmitted signals by a user element 16. Upon arrival of
`the transmitted signals at each of the antennas 40 of the user
`element 16, the respective signals are demodulated and
`amplified by corresponding RF circuitry 74. For the sake of
`conciseness and clarity, only one of the multiple receive
`paths in the receiver is described and illustrated in detail.
`Analog-to-digital (A/D) conversion and downconversion
`circuitry (DCC) 76 digitizes and downconverts the analog
`signal for digital processing. The resultant digitized signal
`may be used by automatic gain control circuitry (AGC) 78
`to control the gain of the amplifiers in the RF circuitry 74
`based on the received signal level.
`The digitized signal is also fed to synchronization cir
`cuitry 80 and a multiple access demodulation function 82,
`which will recover the incoming signal received at a corre
`sponding antenna 40 at each receiver path. The synchroni
`zation circuitry 80 facilitates alignment or correlation of the
`incoming signal with the multiple access demodulation
`function 82 to aid recovery of the incoming signal, which is
`provided to a signaling processing function 84 and channel
`estimation function 86. The signal processing function 84
`processes basic signaling and header information to provide
`information Sufficient to generate a channel quality mea
`Surement, which may bear on an overall signal-to-noise ratio
`for the link, which takes into account channel conditions
`and/or signal-to-noise ratios for each receive path.
`The channel estimation function 86 for each receive path
`provides channel responses corresponding to channel con
`ditions for use by an STC decoder 88. The symbols from the
`incoming signal and channel estimates for each receive path
`are provided to the STC decoder 88, which provides STC
`decoding on each receive path to recover the transmitted
`symbols. The channel estimates provide sufficient channel
`response information to allow the STC decoder 88 to decode
`the symbols according to the STC encoding used by the base
`station 14.
`The recovered symbols are placed back in order using the
`symbol de-interleaver logic 90, which corresponds to the
`symbol interleaver logic 58 of the base station 14. The
`de-interleaved symbols are then demodulated or de-mapped
`to a corresponding bitstream using de-mapping logic 92. The
`bits are then de-interleaved using bit de-interleaver logic 94.
`which corresponds to the bit interleaver logic 54 of the
`transmitter architecture. The de-interleaved bits are then
`processed by rate de-matching logic 96 and presented to
`channel decoder logic 98 to recover the initially scrambled
`data and the CRC checksum. Accordingly, CRC logic 100
`removes the CRC checksum, checks the scrambled data in
`traditional fashion, and provides it to the de-scrambling
`logic 102 for de-scrambling using the known base station
`de-scrambling code to recover the originally transmitted
`data 104.
`A channel quality indicator (CQI) may be determined
`based on the recovered data. An additional or alternative
`CQI function 106 may be provided anywhere along the data
`recovery path (blocks 90 through 104) to monitor signal-to
`noise ratios, error rates, and like to derive information
`bearing on individual or overall channel quality. Additional
`information on one exemplary way to determine a COI value
`is provided in co-assigned application Ser. No. 60/329.511,
`filed Oct. 17, 2001, and entitled “METHOD AND APPA
`
`13
`
`

`

`7
`RATUS FOR CHANNEL QUALITY MEASUREMENT
`FOR ADAPTIVE MODULATION AND CODING.
`The following describes the overall functionality of the
`present invention and refers to the primary device used for
`transmission as the transmitter and the device used for
`receiving as the receiver. At any given time depending on the
`direction of primary communications, the base station 14
`and the user elements 16 may be a transmitter, receiver, or
`both.
`As noted, a MIMO system is one where information is
`10
`transmitted from M transmit antennas and received at N
`receive antennas. As such, there are multiple transmit chan
`nels associated with each antenna. The transfer function for
`each one of these individual channels is represented by h,
`wherein i=1 through N and j=1 through M. The overall
`MIMO system can be expressed by:
`Eq. 1
`y-Hay--n, wherein:
`X-XX. . . X", which represents the channel input;
`y-Iyya. . . yx, which represents the channel output;
`n-Inn. . . ny, which represents channel noise; and
`
`5
`
`15
`
`h11 hi2
`h21 ha2
`H = .
`.
`
`h1M
`h2M
`
`hNi h.N2
`
`hNM
`
`which represents a matrix of the individual channel
`transfer functions corresponding to actual channel con
`ditions.
`From the above, the capacity of the MIMO channel can be
`estimated, such as by using the Shannon equation:
`
`CShannon = log (detly -- HH)bps/Hz,
`
`25
`
`30
`
`35
`
`which is limited by the min{M.N}, wherein I is an identity
`matrix of order N. It is the expected SNR per receiving
`antenna and H is the conjugate transpose of H. For M>N, a
`Maximum Likelihood Decoder may be used to recover
`MIMO signals; however, the complexity of Maximum Like
`lihood Decoding increases exponentially with respect to
`QAM size and the number of transmit antennas, M. For this
`reason, the preferred embodiment limits the number of
`individual BLAST data streams, referred to hereinafter as
`transmitted layers, L, are limited to min M. N. With this
`limitation, Zero forcing, minimum mean square error, or a
`simplified maximum likelihood decoder decoding may be
`used. Such as provided in co-assigned U.S. application Ser.
`No. 10/263,268 filed Oct. 2, 2003 and co-assigned U.S.
`application Ser. No. 10/261,739 filed Oct. 1, 2003, which are
`incorporated herein by reference in their entireties.
`Accordingly, when MDN, the number of layers L is
`limited to the number of receive antennas N(L=N). In a first
`embodiment of the present invention, an NxN subsystem is
`selected from the MXN overall system. In other words, N of
`the M transmit antennas are selected for transmitting the
`respective layers. Preferably, the N transmit antennas are
`selected to maximize system capacity or signal-to-noise
`ratios. The Shannon capacity equation may be used to select
`the most appropriate NxN subsystem, wherein CY repre
`sents the total number of possible Subsystem choices, and
`denotes the number of combinations by taking N transmit
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`US 7,120,395 B2
`
`8
`antennas out of a pool of M transmit antennas. Since the
`channel capacity of a MIMO system depends not only on the
`attenuation associated with the channels but also on the
`actual condition of the channel matrix, the system diversity
`order for the MIMO system is in fact CY, wherein the
`system diversity order is the number of distinct channel
`matrices that can define an NXN MIMO subsystem.
`For a 3x2 MIMO system defined by,
`
`by defining
`
`H12 =
`
`h11 h12
`h21 h22
`
`H13 =
`
`h11 h 13
`h21 h3
`
`H23 =
`
`h12 h13
`h22 has
`
`Eq. 4
`
`and selecting the sub-system H, that satisfies
`Eq. 5
`|det(H)|-max{det(H2), det(His), det(H3)},
`the most robust 2x2 MIMO sub-system, of the three possible
`Sub-systems is selected. In essence, the Subsystem associ
`ated with the most favorable channel conditions is selected
`to transmit the BLAST layers to the user element 16. In this
`embodiment, one of the three transmit antennas is not used
`during transmission. As channel conditions change, the
`unused transmit antenna may change, as illustrated in FIGS.
`6A-6C. Assume that the data within the various BLAST
`layers are represented by S.' and S.'. In FIG. 6A, the data
`S' for the first BLAST layer is transmitted via antenna 40,
`and the BLAST data for the second layer S." is transmitted
`from antenna 40. Transmit antenna 40 will not transmit
`data in association with the other layers. Signals transmitted
`from transmit antennas 40 and 40 are received at both
`receive antennas 28 and 28, and are decoded to recover the
`original data S' and S.'. In FIG. 6B, transmit antenna
`40, is not active, wherein the data S, and S.' are respec
`tively transmitted from transmit antennas 40 and 40s. In
`FIG. 6C, transmit antenna 40, is unused, and the data S'
`and S2' is transmitted from transmit antennas 40, and 40s.
`In this embodiment, N transmit antennas from the pool of M
`transmit antennas are chosen to take advantage of the most
`favorable channel conditions, preferably to either optimize
`throughput, signal-to-noise ratios, or a combination thereof.
`In another embodiment of the present invention, the
`unused or spare transmit antennas of the previous embodi
`ment are used to achieve additional gain for one or more of
`the N layers. In essence, a spare transmit antenna is used to
`redundantly transmit data being transmitted over another of
`the transmit antennas in a manner wherein the primary layer
`information transmitted from a first transmit antenna is
`effectively combined in the channel with the redundantly
`transmitted information from the spare transmit antennas to
`effectively reinforce transmission of the given layer. The
`data transmitted from the spare antenna is weighted to
`achieve a desired gain. The technique of transmitting the
`same data simultaneously from multiple transmit antennas in
`a manner intended to allow the energy of the multiple
`transmitted signals to combine in the channel in a construc
`tive fashion to provide additional gain is referred to as
`beam-forming. In general, one or more of the transmit
`antennas used to transmit a single layer will be associated
`
`14
`
`

`

`in
`
`10
`
`15
`
`then beam-forming can be used to improve the system
`capacity. From the above, the gain from receive antenna
`based beam-forming in an MIMO environment is somewhat
`limited, as the gain in one element of H, does not necessary
`translates into the gain in det(H). However, if beam
`forming is provided at the layer level, the value of det(H)
`can indeed be improved.
`Again, consider a 3x2 MIMO system, and assume that
`H is the sub-system that has been selected a