(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2002/0193146 A1
`
`Wallace et al.
`(43) Pub. Date:
`Dec. 19, 2002
`
`US 20020193146A1
`
`(54) METHOD AND APPARATUS FOR ANTENNA
`DIVERSITY IN A WIRELESS
`COMMUNICATION SYSTEM
`
`(76)
`
`Inventors: Mark Wallace, Bedford, MA (US); Jay
`Rod Walton, Westford, MA (US)
`
`Correspondence Address:
`Sarah Kirkpatrick, Manager
`Intellectual Property Administration
`QUALCOMM Incorporated
`5775 Morehouse Drive
`
`San Diego, CA 92121-1714 (US)
`
`(21) Appl. No.:
`
`09/875,397
`
`(22)
`
`Filed:
`
`Jun. 6, 2001
`
`Publication Classification
`
`Int. Cl.7 ...................................................... H04M 1/00
`(51)
`(52) U.S.Cl.
`......................... 455/562; 455/101; 455/272;
`375/347
`
`ABSTRACT
`(57)
`Method and apparatus for negotiating a transmission sce-
`nario in a mixed mode spectrum Wireless communication
`system capable of both MISO and SISO traffic. The trans-
`mitter determines an antenna diversity configuration for a
`given communication link and applies a transmission sce-
`nario. The base station queries the remote station for antenna
`diversity status. In response to the antenna diversity status
`information, the base station determines and applies a trans-
`mission scenario. In one embodiment, a base station gener-
`ates composite MIMO transmissions to multiple SISO
`mobile stations.
`
`/42
`
`$
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`/44
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`SELECTION UNIT
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`OUTPUT BEST ONE OF
`
`Nr RECEIVERS
`
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`

`Patent Application Publication Dec. 19, 2002 Sheet 1 0f 19
`
`US 2002/0193146 A1
`
`OS
`
`Mag
`
`
`
`|PR2018—01476
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`

`

`Patent Application Publication Dec. 19, 2002 Sheet 2 0f 19
`
`US 2002/0193146 A1
`
`3 F
`
`IG.
`
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`NumberofReceiveAntennas
`
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`Antenna#1
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`

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`Patent Application Publication Dec. 19, 2002 Sheet 3 0f 19
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`US 2002/0193146 Al
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`

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`Patent Application Publication Dec. 19, 2002 Sheet 4 0f 19
`
`US 2002/0193146 A1
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`Patent Application Publication Dec. 19, 2002 Sheet 5 0f 19
`
`US 2002/0193146 A1
`
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`RECEIVERS
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`

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`Patent Application Publication Dec. 19, 2002 Sheet 6 0f 19
`
`US 2002/0193146 A1
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`OUTPUT
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`

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`Patent Application Publication Dec. 19, 2002 Sheet 7 0f 19
`
`US 2002/0193146 A1
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`N‘-
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`IPR2018-01476
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`

`

`Patent Application Publication Dec. 19, 2002 Sheet 8 0f 19
`
`US 2002/0193146 A1
`
`<1:
`
`FIG.
`
`103
`
`212
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`
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`

`

`Patent Application Publication Dec. 19, 2002 Sheet 9 0f 19
`
`US 2002/0193146 A1
`
`252
`
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`
`

`

`Patent Application Publication Dec. 19, 2002 Sheet 10 0f 19
`
`US 2002/0193146 A1
`
`
`
`|PR2018—01476
`
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`

`

`Patent Application Publication Dec. 19, 2002 Sheet 11 0f 19
`
`US 2002/0193146 A1
`
`ENDXE
`
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`
`
`
`
`

`

`Patent Application Publication Dec. 19, 2002 Sheet 12 0f 19
`
`US 2002/0193146 A1
`
`400 \
`
`
`QUERY MOBILE USER FOR DIVERSITY
`CAPABILITY
`
`
`
`
`402
`
`
`
`MIMO
`CAPABLE
`
`LINK
`
`
`
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`SELECT MODE AS
`
`PURE DIVERSITY OR
`
`SPATIAL DIVERSITY
`
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`CONFIGURE
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`CONFIGURE
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`SISO
`CONFIGURE LINK
`MISO
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`fl
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`SIMO
`
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`1111
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`FIG. 13
`
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`
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`

`

`Patent Application Publication Dec. 19, 2002 Sheet 13 0f 19
`
`US 2002/0193146 A1
`
`START
`
`500
`
`
`
`\ QUERY MOBILE USER FOR DIVERSITY
`
`CAPABILITY
`
`
`502
`
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`506
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`CAPABL
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`PURE DIVERSITY OR
`SPATIAL DIVERSITY
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`MISO
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`CONFIGURE
`CONFIGURE
`SISO
`SIMO
`53% m
`
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`
`14
`
`|PR2018—01476
`
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`

`

`Patent Application Publication Dec. 19, 2002 Sheet 14 0f 19
`
`US 2002/0193146 A1
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`(\l
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`
`15
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`CHANNEL
`
`|PR2018—01476
`
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`
`

`

`Patent Application Publication Dec. 19, 2002 Sheet 15 0f 19
`
`US 2002/0193146 A1
`
`TOTRANSMIT
`
`ANTENNAS
`
`
`
`7/708
`
`Wkn
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`IPR2018-01476
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`
`

`

`Patent Application Publication Dec. 19, 2002 Sheet 16 0f 19
`
`US 2002/0193146 Al
`
`
`
`17
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`FIG.
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`
`IPR2018-01476
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`
`

`

`Patent Application Publication Dec. 19, 2002 Sheet 17 0f 19
`
`US 2002/0193146 A1
`
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`Apple Inc. EX1009 Page 18
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`
`
`
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`
`

`

`Patent Application Publication Dec. 19, 2002 Sheet 18 0f 19
`
`US 2002/0193146 A1
`
`1012
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`
`

`

`Patent Application Publication Dec. 19, 2002 Sheet 19 0f 19
`
`US 2002/0193146 A1
`
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`Apple Inc. EX1009 Page 20
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`
`
`
`

`

`US 2002/0193146 A1
`
`Dec. 19, 2002
`
`METHOD AND APPARATUS FOR ANTENNA
`DIVERSITY IN A WIRELESS COMMUNICATION
`SYSTEM
`
`munication link in response to the antenna diversity status,
`and a third set of instructions for applying the first trans-
`mission scenario to the first communication link.
`
`RELATED CO-PENDING APPLICATIONS
`
`[0001] The present Application for Patent is related to
`“METHOD AND SYSTEM FOR INCREASED BAND-
`WIDTH EFFICIENCY IN MULTIPLE INPUT—MUL-
`
`TIPLE OUTPUT CHANNELS” by John Ketchum, having
`US. patent application Ser. No. 09/737,602, filed Jan. 5,
`2001, assigned to the assignee hereof and expressly incor-
`porated by reference.
`
`BACKGROUND
`
`[0002]
`
`1.Ifield
`
`[0003] The present invention relates to Wireless data com-
`munication. More particularly, the present invention relates
`to a novel and improved method and apparatus for antenna
`diversity in a Wireless communication system.
`
`[0004]
`
`2. Background
`
`[0005] To improve the quality of Wireless transmissions,
`communication systems often employ multiple radiating
`antenna elements at the transmitter to communicate infor-
`
`mation to a receiver. Multiple antennas are desirable, as
`Wireless communication systems tend to be interference-
`limited, and the use of multiple antenna elements reduces
`inter-symbol and co-channel interference introduced during
`modulation and transmission of radio signals, enhancing the
`quality of communications. Further,
`the use of multiple
`element antenna arrays at both the transmitter and receiver
`enhances the capacity of multiple-access communication
`systems.
`
`[0006] Each system may employ various antenna configu-
`rations, including user terminals having only single antenna
`capability and other user terminals have multiple antennas.
`Communications for each type of user are processed differ-
`ently. There is a need, therefore, for high-quality, efficient
`communications in a mixed mode system.
`
`SUMMARY
`
`[0007] A method for communication in a Wireless com-
`munication system, the method includes receiving antenna
`diversity status information for a first communication link,
`determining of a configuration of the first communication
`link in response to the antenna diversity status information,
`and applying a transmission scenario to the first communi-
`cation link.
`
`In one aspect, a base station apparatus includes an
`[0008]
`antenna array, and a diversity controller coupled to the
`antenna array, operative for determining a transmission
`scenario based on the configuration of a given communica-
`tion link.
`
`In an alternate aspect, a base station apparatus
`[0009]
`includes a control processor for processing computer-read-
`able instructions, and a memory storage device coupled to
`the control processor, operative to store a plurality of com-
`puter-readable instructions. The instructions include a first
`set of instructions for requesting antenna diversity status of
`the first communication link, a second set of instructions for
`determining a first transmission scenario of the first com-
`
`In still another aspect, a Wireless communication
`[0010]
`system includes a base station, having a first
`receive
`antenna, a first correlator and a second correlator coupled to
`the first receive antenna, a second receive antenna, a third
`correlator and a fourth correlator coupled to the first receive
`antenna, a first combiner coupled to the first and third
`correlators, and a second combiner coupled to the second
`and fourth correlators. According to one embodiment, a first
`code is applied to the first correlator and a second code,
`different
`from the first code,
`is applied to the second
`correlator, the first code is applied to the third correlator and
`the second code is applied to the fourth correlator.
`
`BRHH?DESCRHUTON(H?THE]DRAMHNGS
`
`[0011]
`
`FIG. 1 is a Wireless communication system.
`
`[0012] FIG. 2 is a configuration of transmitter antennas in
`a Wireless communication system.
`
`[0013] FIG. 3 is a table of antenna diversity configura-
`tions in a Wireless communication system.
`FIG. 4 is a mixed mode Wireless communication
`
`[0014]
`system.
`
`[0015] FIG. 5 is a mixed mode Wireless communication
`system.
`
`[0016] FIG. 6 is a model of a channel between transmitter
`and receiver in a Wireless communication system.
`
`[0017] FIG. 7 is model of a channel for a Multiple Input
`Multiple Output, MIMO, configuration.
`
`a Wireless communication system
`[0018] FIG. 8 is
`employing selection diversity at a receiver.
`
`a Wireless communication system
`9 is
`[0019] FIG.
`employing Maximal Ratio Combining, MRC, type selection
`diversity at a receiver.
`
`[0020] FIG. 10 is a Wireless communication system con-
`figured for transmit diversity transmissions.
`
`[0021] FIG. 11 is a Wireless communication system con-
`figured for MIMO transmissions.
`
`[0022] FIG. 12 is a Wireless communication system
`capable of MIMO and diversity transmissions.
`
`[0023] FIG. 13 is a flow diagram of a method of mixed
`mode operation of a forward link in a Wireless communi-
`cation system.
`
`[0024] FIG. 14 is a flow diagram of a method of mixed
`mode operation of a reverse link in a Wireless communica-
`tion system.
`
`[0025] FIG. 15 is a Wireless communication system
`employing transmit diversity.
`
`[0026] FIG. 16 is a Wireless communication system
`employing transmit diversity and spreading codes.
`
`[0027] FIG. 17 is a base station having a distributed
`antenna system for creating multi-paths in a Wireless com-
`munication system.
`
`|PR2018—01476
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`

`US 2002/0193146 A1
`
`Dec. 19, 2002
`
`[0028] FIG. 18 is a base station having a mixed mode
`controller.
`
`one another. In this case it is possible to use SISO for a user
`on one code channel and MISO or MIMO for users on other
`
`[0029] FIG. 19 is a mixed mode wireless communication
`system incorporating MIMO mobile stations and SISO
`mobile stations.
`
`[0030] FIG. 20 is a mobile station adapted for operation
`within a wireless communication system.
`
`DETAILED DESCRIPTION
`
`[0031] The use of multiple element antenna arrays at both
`the transmitter and receiver is an effective technique for
`enhancing the capacity of multiple-access systems. Using
`Multiple Input-Multiple Output, MIMO, the transmitter can
`send multiple independent data streams on the same carrier
`frequency to a user. At high Signal to Noise Ratios, SNRs,
`the increase in throughput approaches N times the through-
`put of single transmit systems operating with Single Input-
`Multiple Output, SIMO, or without receive diversity, Single
`Input-Single Output, SISO, where N=min(Nt,Nr), with NY
`and Nt being the number of receiver and transmitter anten-
`nas, respectively.
`
`In some systems it is desirable to support a mixture
`[0032]
`of user terminal types. For example, terminals designed for
`voice services only may employ a single antenna for receive
`and transmit. Other devices may employ a number of receive
`antennas, and possibly a number of transmit antennas as
`well. To support mixed mode operation the base station must
`be equipped with multiple antennas on which to transmit and
`receive. The table of FIG. 3 gives the matrix of operating
`modes for terminal traffic including SISO, SIMO, Multiple
`Input-Single Output, MISO, and MIMO that can be sup-
`ported by a MIMO capable network.
`
`In multiple access systems it is desirable that all
`[0033]
`four modes of operation be supported. For performance
`reasons it is usually desirable to employ diversity techniques
`(i.e., SIMO and MISO) whenever possible since these
`schemes typically outperform SISO methods. On the uplink,
`also referred to as the reverse link, diversity techniques can
`be supported by placing multiple receive antennas at the
`base stations. On the downlink however, it implies that some
`form of transmit diversity be used when transmitting to
`single receive antenna devices (i.e., MISO). Because MISO
`operation requires different receiver processing than SISO
`operation,
`it is possible that certain systems may have a
`requirement to also support SISO operation for a fraction of
`the terminals.
`
`In Time Division Multiple Access, TDMA, and
`[0034]
`Frequency Division Multiple Access, FDMA, systems it is
`possible to segregate the SISO downlink traffic from the rest
`of the traffic by providing those services on separate time
`slots or frequencies. So, mixed mode operation is relatively
`easy to accommodate in these systems.
`
`In CDMA systems it is not as easy to isolate SISO
`[0035]
`traffic from traffic using other modes. In CDMA systems,
`users are assigned different spreading codes that perform a
`similar function as frequency sub-channels in the FDMA
`case or time slots in the TDMA case. In some cases, the
`spreading codes are designed to be mutually orthogonal so
`that interference from other users is zero. As long as the
`channel is non-dispersive (i.e., no resolvable multipath), the
`orthogonality property holds and users do not interfere with
`
`code channels. However, when the channel becomes time
`dispersive, orthogonality is lost and interference power from
`other users is no longer zero. Channels become dispersive as
`a result of multipath signal propagations that differ from one
`another by more than one spreading chip duration. When
`propagation paths differ by more than one spreading chip in
`duration, they can be independently demodulated using a
`RAKE receiver as is well known in the art and described in
`
`in US. Pat. No. 5,109,390, entitled “Diversity
`detail
`Receiver in a CDMA Cellular Telephone System”, assigned
`to the assignee of the present invention and hereby expressly
`incorporated by reference herein.
`In addition, equalizer
`receiver structures can also be used to demodulate signals
`experiencing multipath propagation.
`
`In traditional CDMA systems, a loss in orthogo-
`[0036]
`nality on the downlink is not necessarily catastrophic since
`the signal and interference terms are correlated on each of
`the delay components. Suppose the channel response is
`given as H0(t)=h0)0(t)+h0)1(t—T), where ho)O is the direct path
`and ho)1 is the reflected path between the transmit antenna 0
`and the user terminal antenna. Further assume that ho)O and
`ho)1 are not highly correlated. The RAKE receiver is essen-
`tially a matched filter in this case, so the average SNR ratio,
`y, can be expressed as:
`
`
`_ w¢10
`11
`,8
`75”" ‘( R )'[q+,310 + 11+1110]’
`
`(1)
`
`[0037] wherein W is the operating bandwidth, R is the data
`rate, I0 is the total power of the downlink, q) is the fraction
`of total power allocated to the user, and n is the thermal
`noise power. Additionally defined are:
`
`a=Ei Ihoplzi’
`
`[0038]
`
`and
`
`l5=Ei |h0,1|2}
`
`(2)
`
`(3)
`
`[0039] wherein E{} signifies expected value. Inspection of
`this SISO SNR expression of equation (1) shows that even
`though the direct and reflected paths of the channel destroy
`orthogonality, they provide a form of implicit diversity. That
`is, the interference power in the denominator of the first term
`in brackets,
`[310,
`is identically correlated with the signal
`power in the numerator of the second term. A similar
`relationship exists for the other path. Assuming the data rate
`and power allocation are matched appropriately, the inter-
`ference power caused by the delay spread does not signifi-
`cantly contribute to the overall error rate. That
`is,
`the
`primary error event is when both paths fade into the noise.
`
`[0040] Now, consider what happens to the SISO receiver
`when another transmit antenna is used to accommodate
`
`users employing MISO and/or MIMO. Using a similar
`channel model as above for the second transmit antenna
`
`results in a channel response of H1(t)=h1)0(t)+h1)1(t—T), and
`the SNR at the RAKE receiver output now becomes:
`
`|PR2018-01476
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`
`.
`Vmeedimode
`
`
`: — +—
`WW.) ) _ [
`0/
`,3
`( R
`,] +1310 +11
`7] + 1110 +11
`
`]
`
`(4)
`
`Inspection of the SISO SNR expression given in
`[0041]
`equation (4) shows that the power from transmit antenna 1,
`11> now present an independent fading interference term in
`the denominator of both terms in the brackets. In this case,
`the primary error event is the desired signal from antenna 0
`fading relative to the interference power emitted from
`antenna 1. So in mixed mode operation (i.e., one transmitter
`communicating with a MIMO and/or MISO user and also
`with a SISO user), the interference power from the addi-
`tional antennas can seriously degrade the performance of
`SISO terminals.
`
`In one embodiment, a CDMA system solves this
`[0042]
`problem using a form of transmit diversity (e.g., MISO) to
`accommodate single receive antenna users when mixed
`mode
`services
`are offered. Various
`alternate MISO
`
`approaches to solving this problem are described herein.
`
`[0043] FIG. 1 is a diagram of a communications system
`100 that supports a number of users and is capable of
`implementing at least some aspects and embodiments of the
`invention. System 100 provides communication for a num-
`ber of cells 102A through 102G, each of which is serviced
`by a corresponding base station 104A through 104G, respec-
`tively. In the exemplary embodiment, some of base stations
`104 have multiple receive antennas and others have only one
`receive antenna. Similarly, some of base stations 104 have
`multiple transmit antennas, and others have single transmit
`antennas. There are no restrictions on the combinations of
`transmit antennas and receive antennas. Therefore,
`it
`is
`possible for a base station 104 to have multiple transmit
`antennas and a single receive antenna, or to have multiple
`receive antennas and a single transmit antenna, or to have
`both single or multiple transmit and receive antennas.
`
`[0044] Terminals 106 in the coverage area may be fixed
`(i.e., stationary) or mobile. As shown in FIG. 1, various
`terminals 106 are dispersed throughout the system. Each
`terminal 106 communicates with at least one and possibly
`more base stations 104 on the downlink and uplink at any
`given moment depending on, for example, whether soft
`handoff is employed or whether the terminal is designed and
`operated to (concurrently or sequentially) receive multiple
`transmissions from multiple base stations. Soft handoff in
`CDMA communications systems is well known in the art
`and is described in detail in US. Pat. No. 5,101,501, entitled
`“Method and system for providing a Soft Handoff in a
`CDMA Cellular Telephone System”, which is assigned to
`the assignee of the present invention and incorporated by
`reference herein.
`
`[0045] The downlink refers to transmission from the base
`station to the terminal, and the uplink refers to transmission
`from the terminal to the base station. In the exemplary
`embodiment, some of terminals 106 have multiple receive
`antennas and others have only one receive antenna. Simi-
`larly, some of terminals 106 have multiple transmit anten-
`nas, and others have single transmit antennas. There are no
`restrictions on the combinations of transmit antennas and
`
`to have multiple transmit antennas and a single receive
`antenna or to have multiple receive antennas and a single
`transmit antenna or to have both single or multiple transmit
`or receive antennas. In FIG. 1, base station 104A transmits
`data to terminals 106A and 106] on the downlink, base
`station 104B transmits data to terminals 106B and 106], base
`station 104C transmits data to terminal 106C, and so on.
`
`the transmitter
`[0046] The use of multiple antennas at
`and/or receiver is referred to as antenna diversity. FIG. 2
`illustrates a physical configuration of multiple antennas at a
`transmitter. The four antennas are each spaced at a distance
`“d” from the next adjacent antenna. The horizontal line gives
`a reference direction. Angles of transmission are measured
`with respect to this reference. The angle “0t” corresponds to
`an angle of a propagation path with respect to the reference
`within a 2-D plane as illustrated. A range of angles with
`respect to the reference is also illustrated. The position and
`angle of propagation define the transmission pattern of the
`antenna configuration. Transmit antenna diversity allows
`directional antennas to form a directed beam for a specific
`user or to form multi-path signals having sufficient separa-
`tion for the receiver to identify the constituent components.
`
`[0047] The receiver may also employ antenna diversity. In
`one embodiment a rake receiver processes multi-path signals
`in parallel, combining the individual signals to form a
`composite, stronger signal. For a given communication link,
`the receiver and/or transmitter may employ some type of
`antenna diversity.
`
`[0048] Diversity reception refers to the combining of
`multiple signals to improve SNR of a system. Time diversity
`is used to improve system performance for 18-95 CDMA
`systems. Generally, buildings and other obstacles in built-up
`areas scatter the signal. Furthermore, because of the inter-
`action between the several incoming waves, the resultant
`signal at the antenna is subject to rapid and deep fading.
`Average signal strength can be 40 to 50 dB below the
`free-space path loss. Fading is most severe in heavily
`built-up areas in an urban environment. In these areas, the
`signal envelope follows a Rayleigh distribution over short
`distances and a lognormal distribution over large distances.
`
`[0049] Diversity reception techniques are used to reduce
`the effects of fading and improve the reliability of commu-
`nication without increasing either the transmitter’s power or
`the channel bandwidth.
`
`[0050] The basic idea of diversity receptions is that, if two
`or more independent samples of a signal are taken, these
`samples will fade in an uncorrelated manner. This means
`that the probability all the samples being simultaneously
`below a given level is much lower than the probability of
`any individual sample being below that level. The probabil-
`ity of M samples all being simultaneously below that level
`is pM, where p is the probability that a single sample is below
`that level. Thus, we can see that a signal composed of a
`suitable combination of the various samples will have much
`less severe fading properties than any individual sample.
`
`In principle, diversity reception techniques can be
`[0051]
`applied either at
`the base station or at mobile station,
`although each type of application has different problems that
`must be addressed. Typically, the diversity receiver is used
`in the base station instead of the mobile station. The cost of
`
`receive antennas. Therefore, it is possible for a terminal 106
`
`the diversity combiner can be high, especially if multiple
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`|PR2018—01476
`
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`
`receivers are required. Also the power output of the mobile
`station is limited by its battery life. The base station,
`however, can increase its power output or antenna height to
`improve coverage to a mobile station. Most diversity sys-
`tems are implemented in the receiver instead of the trans-
`mitter since no extra transmitter power is needed to install
`the receiver diversity system. Since the path between the
`mobile station and the base station is assumed to be approxi-
`mately reciprocal, diversity systems implemented in a
`mobile station work similarly to those in base station.
`
`[0052] A method of resolving multi-path problems uses
`wide band pseudorandom sequences modulated onto a trans-
`mitter using other modulation methods (AM or FM). The
`pseudorandom sequence has the property that time-shifted
`versions are almost uncorrelated. Thus, a signal that propa-
`gates from transmitter to receiver over multi-path (hence
`multiple different time delays) can be resolved into sepa-
`rately fading signals by cross-correlating the received signal
`with multi
`time-shifted versions of the pseudorandom
`sequence. In the receiver, the outputs are time shifted and,
`therefore, must be sent through a delay line before entering
`the diversity combiner. The receiver is called a RAKE
`receiver since the block diagram looks like a garden rake.
`
`[0053] When the CDMA systems were designed for cel-
`lular systems,
`the inherent wide-bandwidth signals with
`their orthogonal Walsh functions were natural for imple-
`menting a RAKE receiver mitigates the effects of fading and
`is in part responsible for the claimed 10:1 spectral efficiency
`improvement of CDMA over analog cellular.
`
`In the CDMA system, the bandwidth (1.25 to 15
`[0054]
`MHZ) is wider than the coherence bandwidth of the cellular
`or Personal Communication System, PCS, channel. Thus,
`when the multipath components are resolved in the receiver,
`the signals from each tap on the delay line are uncorrelated
`with each other. The receiver can then combine them using
`any of the combining schemes. The CDMA system then uses
`the multipath characteristics of the channel to its advantage
`to improve the operation of the system.
`
`[0055] The combining scheme used governs the perfor-
`mance of the RAKE receiver. An important factor in the
`receiver design is synchronizing the signals in the receiver
`to match that of the transmitted signal. Since adjacent cells
`are also on the same frequency with different time delays on
`the Walsh codes, the entire CDMA system must be tightly
`synchronized.
`
`[0056] ARAKE receiver uses multiple correlators to sepa-
`rately detect the M strongest multipath components. The
`relative amplitudes and phases of the multipath components
`are found by correlating the received waveform with
`delayed versions of the signal or vice versa. The energy in
`the multipath components can be recovered effectively by
`combining the (delay-compensated) multipath components
`in proportion to their strengths. This combining is a form of
`diversity and can help reduce fading. Multipath components
`with relative delays of less than At=1/BW cannot be resolved
`and, if present, contribute to fading; in such cases forward
`error—correction coding and power control schemes play
`the dominant role in mitigating the effects of fading.
`
`[0057] Denoting the outputs of the M correlators as Z1,
`Z2,
`.
`.
`.
`, and ZM, and the weights of the corresponding
`outputs as a1, a2, .
`.
`. aM, respectively, the composite signal
`Z is given as
`
`M
`
`k:l
`2:2 ak-Zk.
`
`[0058] The weighting coefficients are based on the power
`or the SNR from each correlator output. If the power or SNR
`is small from a particular correlator, it is assigned a small
`weighting factor. The weighting coefficients, ak, are normal-
`ized to the output signal power of the correlator in such a
`way that the coefficients sum to unity, e.g.,
`
`
`Z?
`
`In CDMA cellular/PCS systems, the forward link
`[0059]
`(BS to MS) uses a three-finger RAKE receiver, and the
`reverse link (MS to BS) uses a four-finger RAKE receiver.
`In the 18-95 CDMA system, the detection and measurement
`of multipath parameters are performed by a searcher-re-
`ceiver, which is programmed to compare incoming signals
`with portions of I- and Q- channel PN codes. Multipath
`arrivals at the receiver unit manifest themselves as correla-
`
`tion peaks that occur at different times. Apeak’s magnitude
`is proportional to the envelope of the path signal. The time
`of each peak, relative to the first arrival, provides a mea-
`surement of the path’s delay.
`
`[0060] The PN chip rate of 1.2288 Mcps allows for
`resolution of multipath components at
`time intervals of
`0.814 us. Because all of the base stations use the same I and
`
`Q PN codes, differing only in code phase offset, not only
`multipath components but also other base stations are
`detected by correlation (in a different search window of
`arrival times) with the portion of the codes corresponding to
`the selected base stations. The searcher receiver maintains a
`
`table of the stronger multipath components and/or base
`station signals for possible diversity combining or for hand-
`off purposes. The table includes time of arrival, signal
`strength, and the corresponding PN code offset.
`
`the base station’s receiver
`[0061] On the reverse link,
`assigned to track a particular mobile transmitter uses the I-
`and Q-code times of arrival to identify mobile signals from
`users affiliated with the that base station. Of the mobile
`
`signals using the same I- and Q-code offsets, the searcher
`receiver at
`the base station can distinguish the desired
`mobile signal by means of its unique special preamble for
`that purpose. As the call proceeds, the searcher receiver is
`able to monitor the strengths of the multipath components
`from the mobile unit to the base station and to use more than
`
`one path through diversity combining.
`
`antenna diversity
`several
`illustrates
`3
`[0062] FIG.
`schemes for a given communication link between a base
`station and a user terminal or mobile station. A communi-
`
`cation link between two transceivers typically includes two
`directional paths, e.g. Forward Link, FL, from a base station
`to a user terminal, and Reverse Link, RL, from the user
`terminal to the base station. Consider one path of a com-
`
`|PR2018-01476
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`
`munication link from a transmitter to a receiver. Four
`
`ter TX antennas. A communication link exists between each
`
`possible configuration types for the path are given in FIG.
`3: Single Input Single Output, SISO; Single Input Multiple
`Output, SIMO; Multiple Input Single Output, MISO; and
`Multiple Input Multiple Output, MIMO. Each configuration
`type describes one path of a given communication link,
`wherein the transmitter for one path is the receiver for the
`other path, and vice versa.
`
`[0063] Note that the number of receive antennas, denoted
`Nr,
`is not necessarily equal
`to the number of transmit
`antennas, denoted Nt, for the transmitter and/or the receiver.
`Therefore, a RL may have a different configuration from that
`of the FL. In practice the base station will not typically
`employ a single transmit antenna, however, with the prolif-
`eration of wireless devices, particularly for voice-only capa-
`bility, single receive antennas at a user terminal are quite
`common.
`
`[0064] A