`6,144,711
`(11) Patent Number:
`United States Patent 5
`Raleigh et al.
`[45] Date of Patent:
`Nov. 7, 2000
`
`
`[54] SPATIO-TEMPORAL PROCESSING FOR
`5,691,727 11/1997 CyZs ceescesssssssseseseseecneesseceneereene 342/361
`COMMUNICATION
`5,752,173
`5/1998 Tsujimoto ...esccecseccseeesseeeseeees 455/137
`5,809,019 9/1998 Ichihara et al.oeseee 370/334
`
`
`esceccsessssessseecsneeeeee 370/329
`5,886,988
`[75]
`Inventors: Gregory G. Raleigh, El Granada;
`3/1999 Yun et al.
`Vincent K. Jones, IV, Redwood
`5,905,721
`5/1999 Liu et al.
`sseeessesseesseesseessesnees 370/342
`Shores; Michael A. Pollack, Cupertino,
`5,966,094 10/1999 Ward et al.
`eeecccceccceeseeneeeen 342/373
`all of Calif.
`
`[73] Assignee: Cisco Systems, Inc., San Jose, Calif.
`[21] Appl. No.: 08/921,633
`[22]
`Filed:
`Aug. 27, 1997
`
`[60]
`
`Related U.S. Application Data
`Provisional application No. 60/025,227, Aug. 29, 1996, and
`provisional application No. 60/025,228, Aug. 29, 1996.
`[SL] Unt, C07 ee cecccseecsssesssssecessseccessneeeseess HO4L 1/02
`[52] U.S. Ch. eeccecccecsssee 375/347; 375/346; 375/349
`[58] Field of Search 0.0.0...eee 375/219, 260,
`375/316, 346, 347, 348, 285, 349, 350;
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`404, 405
`
`[56]
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`
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`6,144,711
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`1
`SPATIO-TEMPORAL PROCESSING FOR
`COMMUNICATION
`
`STATEMENT OF RELATED APPLICATIONS
`
`The present application claims priority from two provi-
`sional applications: SPATIO-TEMPORAL CODING FOR
`WIRELESS COMMUNICATION, U.S. Prov. App. No.
`60/025,227 and SPATIO-TEMPORAL CODING TECH-
`NIQUES FOR RAPIDLY FADING WIRELESS
`CHANNELS, U‘S. Prov. App. No. 60/025,228, both filed on
`Aug. 29, 1996. The contents of both provisional applications
`are herein incorporated by reference for all purposes. These
`applications claim the benefit of U.S. Provisional Applica-
`tion No. 60/025,227, filed Aug. 29, 1997 and U.S. Provi-
`sional Application No. 60/025,228 the disclosure of which is
`incorporated by reference.
`BACKGROUND OF THE INVENTION
`
`The present invention relates to digital communication
`and more particularly to a space-time communication sys-
`tem.
`
`The ability to communicate through wireless media is
`made difficult by the inherent characteristics of how trans-
`mitted signals propagate through the environment. A com-
`munication signal transmitted through a transmitter antenna
`elementtravels along multiple paths to the receiving antenna
`element. Depending on many factors including the signal
`frequency and the terrain, the paths along which the signal
`travels will exhibit different attenuation and propagation
`delays. This results in a communication channel which
`exhibits fading and delay spread.
`It is well knownthat adaptive spatial processing using
`multiple antenna arrays increases the communications qual-
`ity of wireless systems. Adaptive array processing is known
`to improvebit error rate, data rate, or spectral efficiency in
`a wireless communication system. The prior art provides for
`methods involving some form of space-time signal process-
`ing at either the input to the channel,
`the output to the
`channel, or both. The space-time processing step is typically
`accomplished using an equalization structure wherein the
`time domain equalizer tap settings for a multitude of anten-
`nas are simultaneously optimized. This so-called “space-
`time equalizaion”leadsto high signal processing complexity
`if the delay spread of the equivalent digital channel
`is
`substantial.
`
`Thereis priorart teaching the use of conventional antenna
`beams or polarizations to create two or more spatially
`isolated communication channels between a transmitter and
`
`a receiver, but only under certain favorable conditions. The
`radiation pattern cross talk between different physical trans-
`mit and receive antenna pairs must provide sufficientspatial
`isolation to create two or more substantially independent
`communication channels. This can lead to stringent manu-
`facturing and performance requirements on the physical
`antenna arrays as well as the receiver and transmitter elec-
`tronics.
`In addition, when large objects in the wireless
`propagation channel cause multipath reflections, the spatial
`isolation provided by the prior art between any twospatial
`subchannels can be severely degraded, thus reducing com-
`munication quality.
`What is needed is a system for more effectively taking
`advantage of multiple transmitter antennas and/or multiple
`receiver antennas to ameliorate the deleterious effects of the
`inherent characteristics of wireless media.
`
`SUMMARYOF THE INVENTION
`
`The present invention provides a space-time signal pro-
`cessing system with advantageously reduced complexity.
`
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`The system may take advantage of multiple transmitter
`antenna elements and/or multiple receiver antenna elements,
`or multiple polarizations of a single transmitter antenna
`element and/or single receiver antenna element. The system
`is not restricted to wireless contexts and may exploit any
`channel having multiple inputs or multiple outputs and
`certain other characteristics. In certain embodiments, multi-
`path effects in a transmission medium cause a multiplicative
`increase in capacity.
`One wireless embodiment operates with an efficient com-
`bination of a substantially orthogonalizing procedure (SOP)
`in conjunction with a plurality of transmitter antenna ele-
`ments with one receiver antenna element, or a plurality of
`receiver antenna elements with one transmit antenna
`element, or a plurality of both transmitter and receiver
`antenna elements. The SOP decomposes the time domain
`space-time communication channel
`that may have inter
`symbol
`interference (ISI) into a set of parallel, space-
`frequency, SOP bins wherein the ISIis substantially reduced
`and the signal received at a receiver in one bin of the SOP
`is substantially independent of the signal received in any
`other bin of the SOP. Amajorbenefit achieved therebyis that
`the decomposition of the ISI-rich space time channel into
`substantially independent SOP bins makes it computation-
`ally efficient
`to implement various advantageous spatial
`processing techniques embodied herein. Theefficiency ben-
`efit
`is due to the fact
`that
`the total signal processing
`complexity required to optimize performance in all of the
`SOP bins is often significantly lower than the processing
`complexity required to jointly optimize multiple time
`domain equalizers.
`Anotherbenefit is that in many types of wireless channels
`where the rank of the matrix channel that exist between the
`transmitter and the receiver within each SOPbinis greater
`than one, the combination of an SOP with spatial processing
`can be usedto efficiently provide multiple data communi-
`cation subchannels within each SOP bin. This has the
`desirable effect of essentially multiplying the spectral data
`efficiency of the wireless system. A further feature is the use
`of spatial processing techniques within each transmitter SOP
`bin to reduce radiated interference to unintentional receiv-
`ers. A still further feature is the ability to perform spatial
`processing within each receiver SOP bin to reduce the
`deleterious effects of interference from unintentional trans-
`mitters.
`
`One advantageous specific embodiment for the SOPis to
`transmit with IFFT basis functions and receive with FFT
`basis functions. This particular SOP is commonlyreferred to
`as discrete orthogonal frequency division multiplexing
`(OFDM), and each SOP bin is thus associated with a
`frequency bin. This embodiment enhances OFDM with the
`addition of efficient spatial processing techniques.
`According to the present invention, space-frequency pro-
`cessing may adaptively create substantially independent
`spatial subchannels within each SOP bin even in the pres-
`ence of significant cross talk interference between two or
`more physical transmit and receive antenna pairs. A further
`advantageis that the space-frequency processing can advan-
`tageously adaptto cross talk interference between the physi-
`cal antenna pairs even if this cross-talk is frequency
`dependent, or time varying, or both. Thus,
`the present
`invention may provide two or more substantially indepen-
`dent communication channels even in the presence of severe
`multipath and relatively poor physical antenna radiation
`pattern performance.
`A further understanding of the nature and advantages of
`the inventions herein may berealized by reference to the
`remaining portions of the specification and the attached
`drawings.
`29
`
`29
`
`
`
`6,144,711
`
`3
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 depicts a transmitter system according to one
`embodiment of the present invention.
`FIG. 2 depicts a particular substantial orthogonalizing
`procedure (SOP) useful in one embodiment of the present
`invention.
`
`FIG. 3 depicts a receiver system according to one embodi-
`ment of the present invention.
`FIG. 4 depicts a first communication scenario where
`multipath is found.
`FIG. 5 depicts a second communication scenario where
`multipath is found.
`FIG. 6 depicts a third communication scenario where
`multipath is found.
`FIG. 7 depicts a multiple-input, multiple-output (MIMO)
`channel with interference.
`
`FIG. 8 depicts the use of an SOP in a single-input
`single-output (SISO) channel.
`FIG. 9 depicts the use of an SOP in a MIMOchannel
`according to one embodiment of the present invention.
`FIG. 10 depicts the operation of an SOPin the context of
`one embodiment of the present invention.
`FIG. 11 depicts the application of spatial processing to a
`particular SOP bin at the transmitter end according to one
`embodiment of the present invention.
`FIG. 12 depicts the application of spatial processing to a
`particular SOP bin at the receiver end according to one
`embodiment of the present invention.
`FIG. 13 depicts the application of spatial processing to N
`SOPbins at the transmitter end according to one embodi-
`ment of the present invention.
`FIG. 14 depicts the application of spatial processing to N
`SOPbinsat the receiver end according to one embodiment
`of the present invention.
`FIG. 15 depicts the use of a single spatial direction at the
`transmitter end for each bin of an SOP according to one
`embodiment of the present invention.
`FIG. 16 depicts the use of a single spatial direction at the
`receiver end for each bin of an SOP according to one
`embodiment of the present invention.
`FIG. 17 depicts the use of one or more commonspatial
`weighting vectors for all SOP bins at the transmitter end
`according to one embodiment of the present invention.
`FIG. 18 depicts the use of one or more commonspatial
`weighting vectors for all SOP bins at
`the receiver end
`according to one embodiment of the present invention.
`FIG. 19 depicts the use of an encoder for each SOP bin
`according to one embodiment of the present invention.
`FIG. 20 depicts the use of an encoder for each spatial
`direction according to one embodimentof the present inven-
`tion.
`
`FIG. 21 depicts the use of an encoder for each space/
`frequency subchannel according to one embodiment of the
`present invention.
`FIG. 22 depicts distribution of encoder output over all
`space/frequency subchannels according to one embodiment
`of the present invention.
`FIG. 23 depicts a detailed diagram of an encoder/
`interleaver system according to one embodiment of the
`present invention.
`FIG. 24 depicts a transmitter system wherein multiple
`space/frequency subchannels are employed without spatial
`
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`4
`orthogonalization according to one embodiment of the
`present invention.
`FIG. 25 depicts a receiver system wherein multiple space/
`frequency subchannels are employed without spatial
`orthogonalization according to one embodiment of the
`present invention.
`FIG. 26 depicts an exemplary technique for bit loading
`with a trellis coder that uses a one-dimensional QAM
`symbol constellation.
`DESCRIPTION OF SPECIFIC EMBODIMENTS
`Definitions
`A “channel” refers to the input symbol to output symbol
`relationship for a communication system. A “vector chan-
`nel” refers to a channel with a single input and multiple
`outputs (SIMO), or multiple inputs and a single output
`(MISO).Each h, entry in the vector channelh describes one
`of the complex path gains present in the channel. A “matrix
`channel” refers to a channel with multiple inputs and mul-
`tiple outputs (MIMO). Each entry H,; in the matrix H
`describes the complex path gain from input j to output 1. A
`“space time channel” refers to the input to output relation-
`ship of a MIMO matrix channel, or a SIMOor MISO vector
`channel, that occurs when multipath signal propagation is
`present so that the channel contains delay elements that
`produce inter-symbolinterference (ISI) as explained below.
`A “spatial direction”is a one dimensional subspace within
`a matrix or vector communication channel. Spatial direc-
`tions need not be orthogonal. A spatial direction is typically
`characterized by a complex input vector and a complex
`output vector used to weight transmitted or received signals
`as explained herein.
`A “sub-channel” is a combination of a bin in a substan-
`tially orthogonalizing procedure (SOP) as explained below
`and a spatial direction within that bin. A group of spatial
`subchannels within an SOP bin may or may not be orthogo-
`nal.
`
`An “orthogonal dimension” is one member in a set of
`substantially orthogonal spatial directions.
`Achannel“subspace”is a characterization of the complex
`m-space direction occupied by one or more m-dimensional
`vectors. The subspace characterization can be based on the
`instantaneousor average behavior of the vectors. A subspace
`is often characterized by a vector-subspace of a covariance
`matrix. The covariance matrix is typically a time or fre-
`quency averaged outer product of a matrix or vector quan-
`tity. The covariance matrix characterizes a collection of
`average channel directions and the associated average
`strength for each direction.
`A“two norm”metric for a vectoris the sum of the squared
`absolute values for the elements of the vector.
`
`A “Euclidean metric” is a two norm metric. “Intersymbol
`interference” (ISDrefers to the self interference that occurs
`between the delayed and scaled versions of one time domain
`symbol and subsequent symbols received at the output of a
`delay spread communication channel. The channel delay
`spread is caused by the difference in propagation delay
`between the various multipath components combined with
`the time domain response of the RF and digital filter
`elements.
`A “substantially orthogonalizing procedure” (SOP) is a
`procedure that plays a part in transforming a time domain
`sequence into a parallel set of substantially orthogonal bins,
`wherein the signals in one bin do not substantially interfere
`with the signals from other bins. Typically, the transforma-
`tion from a time domain sequence to a set of substantially
`orthogonal bins requires a transmitter SOP with a set of
`input bins, and a receiver SOP with a set of output bins.
`30
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`6,144,711
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`6
`numberof bits that are mapped to a given encoder symbol,
`and the signal powerassignedto that symbol, are determined
`based upon the measured communication quality of the
`space-frequency information subchannel
`that carries the
`symbol stream.
`After the digital data is encoded into a sequence of
`symbols, a Training Symbol Injection block 20 may be used
`to place a set of known training symbol values in the
`transmitter symbol stream. The purpose of the training
`symbols is to provide a known input within a portion of the
`transmitted symbol stream so that a receiver may estimate
`the communication channel parameters. The channel esti-
`mate is used to aid in demodulation and decoding of the data
`sequence. The training symbols maybe injected periodically
`in time, periodically in frequency, or both. It will be obvious
`to one skilled in theart that blind adaptive spatial processing
`techniques can be utilized within each SOP bin at
`the
`receiver as an alternative to tra