(12)
`
`United States Patent
`Nagaraj
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,054,664 B2
`May 30, 2006
`
`USOO7054664B2
`
`(54) METHOD AND APPARATUS FOR
`PROVIDING USER SPECIFC DOWNLINK
`BEAMFORMING IN A FIXED BEAM
`NETWORK
`
`(75) Inventor: Shirish Nagaraj, Middletown, NJ (US)
`
`(73) Assignee: Lucent Technologies Inc., Murray Hill,
`NJ (US)
`
`(*) Notice:
`
`-
`0
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 256 days.
`
`(21) Appl. No.: 10/696,930
`
`(22) Filed:
`
`(65)
`
`Oct. 30, 2003
`9
`Prior Publication Data
`US 2005/OO96090 A1
`May 5, 2005
`(51) Int. Cl.
`(2006.01)
`H04O 7/20
`(52) U.S. Cl. .................... 455/562.1; 455/25; 455/634;
`455/69; 455/103; 455/121; 455/193.1; 455/575.7;
`342/154; 342/367; 342/368; 342/373; 342/417
`(58) Field of Classification Search .................. 455/25,
`455/634, 121, 193.1, 276, 273, 440,562. 1,
`455/575.7, 69, 103,561; 342/154, 157,359,
`342/367–368, 373, 417
`See application file for complete search history.
`References Cited
`
`(56)
`
`U.S. PATENT DOCUMENTS
`5,045,859 A * 9, 1991 Yetter ......................... 342/414
`5,634,199 A * 5/1997 Gerlach et al. .....
`... 455,631
`6,130,643 A * 10/2000 Trippett et al. ............. 342,380
`6,345,188 B1* 2/2002 Keskitalo et al. ........... 455,561
`
`210
`
`USERKS
`SIGNALS
`
`8/2002 Kikuchi ...................... 342,368
`6,433,738 B1
`6,470,194 B1 * 10/2002 Miya et al. .......
`... 455,562.1
`6,477,161 B1 * 1 1/2002 Hudson et al. ............. 370,342
`6,606,058 B1* 8/2003 Bonek et al. .....
`... 342,383
`6,876,693 B1 * 4/2005 Sim .................
`... 375,144
`2002fOO68590 A1* 6, 2002 Suzuki et al. ..
`... 455,466
`2003/0048760 A1* 3, 2003 Park et al. .....
`... 370,295
`2003/0092469 A1* 5, 2003 Takano ......
`... 455,562
`2003/O124994 A1* 7, 2003 SR
`... 455.91
`2003. O151553 A1* 8, 2003 Yitalo ......
`... 342/422
`2003/0152099 A1
`8/2003 Chun et al. ................. 370,441
`2003/0162567 A1* 8/2003 Raghothaman et al. ..... 455,562
`2003/O190897 A1* 10, 2003 Lei et al. .................... 455,101
`2004/00 14499 A1
`1/2004 Hamalainen et al. ....... 455,561
`2004/00 14501 A1
`1/2004 Kuwahara et al. .......... 455,561
`2004/0063468 A1* 4/2004 Frank ...............
`... 455,561
`2004/0082299 A1
`4/2004 Brunner et al. ............. 455,101
`2004/0157646 A1* 8/2004 Raleigh et al. .......... 455,562.1
`2005/0101352 A1* 5/2005 Logothetis et al. ...... 455,562.1
`* cited by examiner
`Primary Examiner William Trost
`Assistant Examiner James D. Ewart
`(57)
`ABSTRACT
`
`Ele disclosed embodiments relate to a system and method
`or providing user specific beams in a fixed beam network,
`the fixed beam network comprising a plurality of fixed
`beams, each of the plurality of fixed beams being defined by
`a plurality offixed beam correlation coefficients. The system
`may comprise a device that computes reception correlation
`data for a received signal, and a beam former that is adapted
`to determine transmission weighting coefficients to be
`applied to a return signal based on the difference between the
`reception correlation data and the fixed beam weighting
`coefficients associated with at least one of the plurality of
`fixed beams.
`
`19 Claims, 4 Drawing Sheets
`
`CALCULATE
`BEAMFORMING
`WEIGHTw,
`
`-220
`CALCULATER(1)
`and p()
`
`222
`
`COMPUTER(1)
`FROMUPLINK
`PILOT
`
`
`
`VWGoA EX1012
`U.S. Patent No. 10,965,512
`
`

`

`U.S. Patent
`
`May 30, 2006
`
`Sheet 1 of 4
`
`US 7,054,664 B2
`
`28
`
`24
`
`26
`
`16
`
`22
`
`2O
`
`12
`
`BEAMFORMER
`
`
`
`
`
`PROCESSING
`CIRCUITRY
`
`14
`
`

`

`U.S. Patent
`U.S. Patent
`
`May 30, 2006
`May 30, 2006
`
`Sheet 2 of 4
`Sheet 2 of 4
`
`US 7,054,664 B2
`US 7,054,664 B2
`
`
`
`
`
`

`

`U.S. Patent
`
`May 30, 2006
`
`Sheet 3 of 4
`
`US 7.054,664 B2
`
`O
`
`NY
`
`2 O 2 9. 11 2
`22 OO 64
`22 11 64
`317
`
`210
`
`X
`USER KS 1. 212 NY
`SIGNALS
`2.
`214Y
`tex
`2
`Ox 216Y
`los
`CALCULATE --218
`BEAMFORMING
`WEIGHTV
`
`CALCULATER(1) 220
`and p()
`
`COMPUTER(I) 222
`FROM UPLINK
`PILOT
`
`2OO
`
`mrr.
`
`FIG. 3
`
`

`

`U.S. Patent
`
`May 30, 2006
`
`Sheet 4 of 4
`
`US 7,054,664 B2
`
`
`
`ANTENNA
`
`BEAM
`COMPUTING
`UNIT
`
`FIG. 4
`
`

`

`US 7,054,664 B2
`
`1.
`METHOD AND APPARATUS FOR
`PROVIDING USER SPECIFC DOWNLINK
`BEAMFORMING IN A FIXED BEAM
`NETWORK
`
`BACKGROUND OF THE INVENTION
`
`2
`received signal. The phase reference to be used by a mobile
`transceiver for a given communication session is typically
`specified by Radio Resource Control (RRC, or upper layer
`protocol) signaling. In UMTS and other systems, available
`phase references may include common pilot channels such
`as the primary common pilot channel (P-CPICH) and the
`secondary common pilot channel (S-CPICH). Another pilot
`channel, which may be referred to as a dedicated pilot
`channel (DPILOT) may be provided as well.
`Because the mobile transceiver typically has no aware
`ness that any type of beam forming is being applied (because
`of the proprietary nature of base station antenna configura
`tions), the type of phase-reference that the mobile uses for
`its channel estimation and signal demodulation is an impor
`tant aspect of the performance of the beam forming algo
`rithm. Thus, beam formers have to be designed keeping in
`mind the phase-reference that a mobile transceiver is going
`tO use.
`Downlink beam forming is a method of signal transmis
`sion from a group of closely spaced antennas, such as a
`cellular telephone base station. Transmission from the base
`station may be designed such that the signals transmitted to
`a mobile transceiver all arrive co-phased at the mobile
`antenna. Because of the closely spaced nature of the base
`station antennas, the wireless channels from the base station
`antennas to the mobile antenna are all highly correlated. This
`correlation is represented by a spatial correlation matrix,
`which can be measured from uplink pilot signals. The spatial
`channel correlation is exploited by a beam former, which
`applies appropriate complex weights to the signal at the
`different antenna elements. The weights may be designed
`such that a particular user's signals from all the transmitting
`antennas, after going through the channels, arrive coherently
`(or co-phased) at the user's receiving antenna. This typically
`results in a signal-to-interference ratio improvement of
`about a factor equal to the number of transmit antennas. As
`Such, the design of a beam former is an important element
`with respect to the performance of any multi-antenna sys
`tem.
`There are two types of beam formers that may be
`employed in a multi-antenna wireless system. One is the
`user-specific beam former, which forms beams on a per user
`basis, one beam per user. This requires information on the
`user's channel characteristics, typically obtained through the
`spatial channel correlation matrix, which is computed based
`on an uplink pilot received when the mobile transceiver
`sends data to the base station. This correlation matrix
`essentially gives a measure of the direction in which the UE
`is located, which may allow a beam former to form beams
`that point in that direction.
`User specific beam forming has several shortcomings,
`however. User specific beam forming may employ the
`P-CPICH for channel estimation and synchronization pur
`poses, but the P-CPICH may only be effective as a phase
`reference under certain conditions, such as when employed
`in Systems with very few closely spaced antennas.
`However, user specific beam forming strategies that are
`appropriate for systems that employ a very Small number of
`closely spaced antennas may not work well when extended
`to systems that have many closely spaced antennas. This is
`because the correlation across the antennas decreases as the
`channel becomes more spatially scattered (leading to what is
`called as high angular spread). The problem of high angular
`spread arises when there are a large number of spatially
`dispersed local scatterers around the mobile user. Beam
`forming becomes less effective when angular spread is high
`because the signal energy arrives only partially co-phased at
`
`10
`
`15
`
`30
`
`35
`
`40
`
`1. Field of the Invention
`The present invention relates generally to communication
`systems that transmit, receive and process communication
`signals and, more particularly, to providing user specific
`downlink beam forming in a fixed beam network.
`2. Description of the Related Art
`This section is intended to introduce the reader to various
`aspects of art that may be related to various aspects of the
`present invention, which are described and/or claimed
`below. This discussion is believed to be helpful in providing
`the reader with background information to facilitate a better
`understanding of the various aspects of the present inven
`tion. Accordingly, it should be understood that these state
`ments are to be read in this light, and not as admissions of
`prior art.
`Communication systems that transmit and receive com
`munication signals continue to grow in importance. Such
`systems are used to provide television, radio, satellite com
`25
`munication, cell phone service, wireless computing net
`works and the like. An important aspect of Such systems is
`the ability to efficiently process signals to continue to
`improve the quality of service provided to users.
`Antenna arrays may be used to perform beam forming to
`enhance reception of signals from different angles of arrival,
`and transmit beamforming to enhance the quality of trans
`mission of signals to different users. Phase offsets between
`signals received from a user on different elements of the
`antenna array depend on the angle of arrival of the user's
`signals at the antenna array. This phenomenon can be
`utilized to combine signals arriving from a desired direction
`constructively at the base station receiver using a receive
`beam former. A receive beam former is a device that receives
`inputs from the various elements of an antenna array and
`combines them into output signals or beams based on certain
`criteria.
`In addition, transmit beam formers may be used to
`enhance signals prior to their transmission by an antenna
`array. Transmit beam formers may apply weighting coeffi
`cients to the signal intended for any user before transmission
`by an antenna array Such that the desired signal strength for
`the user is enhanced and/or that the interference caused by
`this user's signal to other users is reduced. The weighting
`coefficients applied by a transmit beam former may be
`adjusted according to various measurements of the signals
`received from the desired user at the antenna array or any
`other knowledge of the users angle of location from the
`antenna array. Using transmit beam forming weight coeffi
`cients, the signal intended for a desired user may be thought
`ofas being “steered toward the direction of the desired user,
`Such that the signals strength for the desired user is maxi
`mized and interference caused by this signal to users located
`at other angles is reduced.
`Mobile transceivers, such as cellular telephone handsets,
`may be referred to as user equipment (UE). Channel esti
`mation by a mobile transceiver is very important to realize
`beam forming gains. In many wireless communication sys
`tems, including third generation (3G) systems such as Uni
`versal Mobile Telephone Service (UMTS), phase reference
`signals may be provided to assist mobile transceivers in
`performing channel estimation and synchronizing on a
`
`55
`
`45
`
`50
`
`60
`
`65
`
`

`

`US 7,054,664 B2
`
`10
`
`15
`
`25
`
`30
`
`35
`
`3
`the mobile antenna. Further, the fact that the P-CPICH
`phase-reference is not beam formed leads to much steeper
`degradation in performance because the pilot and traffic see
`different channels as angular spread increases. This is known
`as pilot-to-traffic mismatch and beam forming systems must
`perform within extremely strict tolerances to optimally
`realize beam forming benefits in the face of such mis
`matches.
`The other typical type of beam forming system is known
`as a fixed beam forming system. In fixed beam forming
`systems, the base station does not form beams appropriate
`for each and every user, but rather forms a set of few
`common beams pointing in different predetermined direc
`tions, such that the whole cell area of interest is covered.
`These common beams are called fixed because they do not
`adapt to any particular user's location. However, these
`common beams can be made to change from time to time
`depending on various factors. Such as changes in traffic load
`pattern and the like. As long as the beams are not designed
`to serve any one particular user, but rather meant to serve a
`common cellular Sub-area, that type of system is referred to
`a fixed beam forming network.
`Fixed beam forming does not suffer from the pilot-to
`traffic mismatch problem because the common pilot may be
`a secondary common pilot channel (S-CPICH), which may
`be sent over the same fixed beam that is used to serve a user.
`A user who happens to be at the peak of a beam being used
`for signal transmission may see the maximum possible
`beam forming benefit. A steep decline in performance
`because of steep roll-off of the beam patterns may be
`experienced by users that are between two beams. This
`performance decline may be on the order of around three (3)
`dB for a four (4) antenna base-station.
`Fixed beam forming, therefore, is intrinsically not fair,
`because users get different quality of service (QoS) depend
`ing on their geographic location. This situation is clearly
`undesirable. These “coverage gaps’ can be minimized by
`defining more fixed beams, but defining more fixed beams
`entails an increased power allocation for the overhead
`channels. This is true because correspondingly more sec
`ondary common pilot channels S-CPICHs would be needed.
`Another option is to Sweep the beams periodically in time.
`However, this strategy trades the performance losses among
`the different users in time and does not alleviate the problem
`of performance loss completely.
`Further, if a user moves from the coverage area of one
`beam to another, there is a typical delay in signaling the user
`to change its phase-reference (S-CPICHID) because higher
`layer signaling is involved. Because of this delay, the user
`could continue to use an “old” phase reference for some
`time, resulting in a much greater degradation of perfor
`mance. This is an important problem since the beams have
`a very sharp decline in gain in areas beyond their main
`coverage areas.
`
`4
`reception correlation data and the fixed beam weighting
`coefficients associated with at least one of the plurality of
`fixed beams.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`Advantages of the invention may become apparent upon
`reading the following detailed description and upon refer
`ence to the drawings in which:
`FIG. 1 is a block diagram illustrating a communication
`system in accordance with embodiments of the present
`invention;
`FIG. 2 is a block diagram showing the deployment of
`fixed beams in a fixed beam network in accordance with
`embodiments of the present invention;
`FIG. 3 is a schematic diagram showing the operation of a
`beam former in accordance with embodiments of the present
`invention; and
`FIG. 4 is a block diagram showing a mobile transceiver in
`accordance with an alternative embodiment of the present
`invention.
`
`DETAILED DESCRIPTION OF SPECIFIC
`EMBODIMENTS
`
`One or more specific embodiments of the present inven
`tion will be described below. In an effort to provide a concise
`description of these embodiments, not all features of an
`actual implementation are described in the specification. It
`should be appreciated that in the development of any such
`actual implementation, as in any engineering or design
`project, numerous implementation-specific decisions must
`be made to achieve the developers specific goals, such as
`compliance with system-related and business-related con
`straints, which may vary from one implementation to
`another. Moreover, it should be appreciated that such a
`development effort might be complex and time consuming,
`but would nevertheless be a routine undertaking of design,
`fabrication, and manufacture for those of ordinary skill
`having the benefit of this disclosure.
`Embodiments of the present invention may form user
`specific beams in the presence of a fixed beam forming
`network. If the network is configured to Support secondary
`common pilot channels (S-CPICHs), which are transmitted
`with fixed beam forming weights, the users can be made to
`use one of the S-CPICHs as a phase reference for demodu
`lating their signal. In that case, sending the user's signals
`with the same fixed beam forming weights as their reference
`S-CPICH channel is straightforward, but entails loss of
`performance because of the mismatch between the fixed
`beam forming weights and the channel seen by that user.
`An improved strategy for providing user specific beam
`forming in a fixed beam forming network may employ the
`principle that the phase reference is a known fixed beam
`forming weight vector, and thus is employed to calculate
`good dedicated weights for each user, taking into account the
`fact that the mobile transceiver has no knowledge that
`beam forming is taking place. Embodiments of the present
`invention may utilize the user-specific channel correlation
`matrix information from uplink measurements to steer the
`user's signal in the direction of the mobile transceiver.
`Referring to the drawings, FIG. 1 is a block diagram
`illustrating a communication system in accordance with
`embodiments of the present invention. The communication
`system, which may comprise a cellular base station, is
`generally referred to by the reference numeral 10. A beam
`former 12 is connected to receive and transmit signals from
`
`40
`
`45
`
`50
`
`55
`
`SUMMARY OF THE INVENTION
`
`The disclosed embodiments relate to a system and method
`for providing user specific beams in a fixed beam network,
`the fixed beam network comprising a plurality of fixed
`beams, each of the plurality of fixed beams being defined by
`a plurality of fixed beam weighting coefficients. The system
`may comprise a device that computes reception correlation
`data for a received signal, and a beam former that is adapted
`to determine transmission weighting coefficients to be
`applied to a return signal based on the difference between the
`
`60
`
`65
`
`

`

`US 7,054,664 B2
`
`5
`
`10
`
`15
`
`5
`a plurality of antenna elements 16, 18 and 20. The antenna
`elements 16, 18 and 20 present an antenna pattern 22. The
`antenna pattern 22 is illustrative of the directions from
`which the antenna is likely to have the best reception of
`transmitted communication signals.
`The antenna pattern 22 may comprise lobes, such as the
`lobes 24 and 28. Additionally, the antenna pattern may
`comprise troughs such as the trough 26. When processing
`received communication signals, the beam former 12 may be
`adapted to provide fixed beams by employing weighting
`coefficient for received signal components indicative of a
`plurality of predefined directions. The beam former 12 may
`employ dynamically controllable weighting coefficients to
`mathematically give greater weight to signals received from
`the predefined directions when producing its output signal.
`The beam former 12 is connected to processing circuitry
`14, which may perform processing on communication sig
`nals after they are received or prior to being transmitted. As
`set forth above, the beam former may be intended to provide
`a fixed beam communication network. Additionally, the
`beam former 12 may be adapted to provide individualized
`beams to a plurality of users even though those users may
`not be in the general proximity of one of the predefined
`beams of the system 10. The use of the beam former 12 to
`provide a fixed beam network is discussed below with
`respect to FIG. 2.
`FIG. 2 is a block diagram showing the deployment of
`fixed beams in a fixed beam network in accordance with
`embodiments of the present invention. The fixed beam
`network is generally referred to by the reference numeral
`100. As illustrated in FIG. 2, embodiments of the present
`invention relate to the incorporation of user specific beam
`forming in a fixed beam network in which common pilot
`synchronizing signals are sent over the fixed beams. The
`optimal user-specific open-loop beam former when the
`demodulation of the signal is performed via a pre-deter
`mined fixed beam phase reference is derived. The beam
`former 12 (FIG. 1) may be applicable for an arbitrary
`antenna configuration without any assumptions on the chan
`nel covariance matrix. By way of example, a three-element
`antenna array is illustrated in FIG. 2.
`In FIG. 2, the fixed beam network 100 is established by
`a three-element antenna array that comprises antenna ele
`ments 102, 104 and 106. The antenna array may comprise a
`45
`portion of a cellular telephone base station or the like. A
`beam former such as the beam former 12 (FIG. 1) is adapted
`to provide three fixed beams 110, 112 and 114. Each of the
`fixed beams 110, 112 and 114 provide a coverage envelope
`where reception is the strongest. In FIG. 2, the fixed beams
`110, 112 and 114 respectively provide coverage envelopes
`118, 120 and 122.
`The fixed beam system 100 may include circuitry or
`Software (not shown) that assigns a particular beam to a
`particular user when the user is in the proximity of one of the
`coverage envelopes 118, 120 or 122. When a user is between
`one of the fixed beams, the user's reception would be
`degraded compared to when the user is within one of the
`coverage envelopes 118, 120 or 122.
`The following discussion employs UMTS terminology
`and assumes the pilot structure to be available as in UMTS,
`although the concepts developed are applicable in principle
`to any wireless system that allows for Such pilot configu
`rations. In wireless communication systems, pilot channels
`provide a known phase reference synchronization signal that
`may be used by a mobile transceiver to locate data elements
`within the transmitted data packets.
`
`55
`
`6
`In UMTS systems, there are three standard phase-refer
`ences that may be employed by a mobile transceiver to
`perform channel estimation and assist in signal synchroni
`Zation. One is the primary common pilot channel
`(P-CPICH), which is typically a common pilot transmitted
`over the whole cell of interest by a single antenna.
`The second type of pilot that may be employed as a phase
`reference signal is the secondary common pilot channel
`(S-CPICH). More than one S-CPICH may be employed in
`each cell. As set forth below, the S-CPICH may be beam
`formed with fixed beams, with one pilot going out per beam.
`At the time of call initiation, the user's S-CPICH ID may be
`determined based on that user's uplink signal. The base
`station sends certain signal strength measurements to the
`Radio Network Controller (RNC), which then decides which
`beam is ideally suited for that user (if that user is configured
`to use S-CPICH as phase-reference). Then, knowing the
`mapping between the beams and the S-CPICH ID, the RNC
`may convey both to the user and to the base station, the
`S-CPICH ID that the user is going to use for its demodu
`lation. The beam corresponding to this S-CPICH ID may be
`termed as the serving beam for that user. The RNC can also
`decide to switch the S-CPICH ID (or equivalently, the
`serving beam) if uplink measurements indicate that the user
`has moved into the area of another beam.
`The third pilot that may be used for phase reference
`purposes is the dedicated pilot (DPILOT), which is time
`multiplexed with the user's data signal. At first glace, the
`DPILOT seems to be the best choice as far as phase
`reference is concerned in beam formed systems because the
`DPILOT is also beam formed with the same weights that the
`data is transmitted with, resulting in a perfectly matched
`channel estimate. However, the DPILOT is a very weak pilot
`and the number of symbols available for channel estimation
`is also limited, leading to a very noisy channel estimate.
`Thus, DPILOT is very unreliable and cannot be used for
`beam formed systems. Simulations have shown that using
`the DPILOT as phase-reference results in much degraded
`beam forming performance.
`Each of the fixed beams 110, 112 and 114 in FIG. 2 are
`shown as incorporating secondary common pilot channels
`(S-CPICHs) therein. The fixed beam 110 incorporates sec
`ondary common pilot channels 126, 128, 130 and 132,
`which maybe used for synchronization by user equipment.
`The use of the secondary common pilot channels 126, 128,
`130 and 132 typically improves user reception and does not
`result in traffic to pilot mismatch. The fixed beams 112 and
`114 are illustrated to have secondary common pilot channels
`incorporated therein, but those secondary common pilot
`channels are not given reference numerals for purposes of
`simplifying FIG. 2.
`As set forth above, embodiments of the present invention
`may employ beam steering to create user specific beams,
`providing a benefit accrued by sending the user's signals
`over the same fixed beam that the user is configured to
`demodulate as its phase reference (for example, the second
`ary common pilot channel). For example, if a mobile user
`126 is between the coverage envelopes 118 and 120 pro
`vided by the fixed beams 110 and 112, as shown in FIG. 2,
`a beam former may be adapted to provide a user specific
`beam 128 directed at the mobile user 126 to improve the
`user's reception.
`As set forth above, embodiments of the present invention
`may create user-specific beams in the presence of a fixed
`beam forming network. If the network is configured to Sup
`port S-CPICHs, which are transmitted with fixed beam form
`ing weights, the users can be made to use one of the
`
`25
`
`30
`
`35
`
`40
`
`50
`
`60
`
`65
`
`

`

`US 7,054,664 B2
`
`8
`The best beam that is used to serve as a phase-reference
`for that user is determined as follows:
`
`L.
`
`m = arg, max, 2, W. Rhh (l)wk
`
`(4)
`
`Using this, the user is signaled via RRC layer signaling to
`use the S-CPICH ID that corresponds to the fixed beam m.
`The optimal beam former V is computed as a weighted
`linear combination of the fixed beam weight vectors as
`follows:
`
`(5)
`
`Further, a “virtual channel vector may be defined as:
`
`with a correlation matrix given by:
`(7)
`R(I)=WR, (2) W*
`The decision statistic for the detection of s at the mobile,
`using the channel estimate from the S-CPICH, is given by
`
`L.
`
`=
`
`(8)
`
`where g(i) is the m" component of g(i) and the S-CPICH
`channel estimate is assumed to be ideal.
`This virtual channel may be expressed in terms of the
`phase-reference channel estimate g(i), as:
`g(i) g, (i)priq.(i)
`with EIg(i)*q.(i)=0
`Let p(l)-Egg, and O(ml)-Egy. Then
`define
`
`(9)
`
`pm (l)
`pi F or (m, 1)
`
`1
`
`p.m (l) T'
`O(m, l)
`
`(10)
`
`whR. (1)w,
`
`w, Ru(l)w,
`
`where the unity appears in the m” position in the first
`equation. Define the Sum correlation matrix of the Zero
`mean complex-valued Gaussian distributed vector q overall
`all paths, as:
`
`7
`S-CPICH's as a phase reference for demodulating their
`signal. In that case, sending the users signals with the same
`fixed beam forming weights as their reference S-CPICH
`channel is straightforward, but entails loss of performance
`due to the fact that none of the fixed beams is optimally
`appropriate for a particular user.
`Embodiments of the present invention may employ the
`principle that dedicated weights for each user can be com
`puted because the phase reference signal (S-CPICH) is a
`10
`channel that is seen from a known fixed beam forming
`weight vector. This computation implicitly accounts for the
`fact that the mobile has no knowledge of any such beam
`forming being applied. This beam former utilizes the user
`specific channel correlation matrix information from uplink
`measurements to steer the user's signal in the direction of the
`user's mobile transceiver. The beam former then determines
`transmission weighting coefficients to be applied to a return
`signal based on the difference between the reception corre
`lation data and the fixed beam weighting coefficients asso
`ciated with the S-CPICH that has been designated for the
`user's communication session.
`The following discussion illustrates how a user specific
`beam former is determined in the context of a fixed beam
`network environment. On the downlink, it may be assumed
`that the signal for a user is beam formed with M closely
`spaced transmitting antennas. The beam formed is given by
`an M -dimensional complex valued weight vector V. Let the
`1" multipath channel from the transmit antennas to the
`mobile receive antenna be denoted by h, where l=1,2,...,
`L. As used herein, x, x' and x * denote the conjugate,
`transpose and Hermitian operations for a vector X.
`Further, {} and s () denote the real and imaginary parts
`of a complex-valued entity. M fixed beams are defined,
`given by weight matrix W-w, w, .
`.
`. .w
`with the
`property that:
`wif w=0 ifizi and
`
`15
`
`25
`
`30
`
`35
`
`|w,’=1 i=1,2,....M
`(1)
`Each fixed beam vector w, is used to carry one S-CPICH
`pilot.
`The received signal at the mobile, for the 1" multipath
`channel and the i' symbol instance, after despreading with
`the scrambling and channelization codes, is given by:
`y(i)=VPv'h.(i)s(i)+n,(i)
`
`(2)
`
`40
`
`45
`
`50
`
`55
`
`Here, s(i) is the binary information symbol to be transmitted,
`P=NE, where N is the spreading gain of a code division
`multiple access (CDMA) signal and E is the transmit energy
`per chip for that user. The random interference plus noise
`component n(i) is complex-valued Zero-mean Gaussian
`distributed with variance equal to o, . Further, h(i) is a
`Zero-mean complex Gaussian channel with correlation
`matrix R(1) that can be estimated from uplink measure
`ments. The reception correlation data that comprises the
`correlation matrix R(1) is obtained from uplink commu
`60
`nications from a mobile transceiver and may be based on
`received pilot signals transmitted by the mobile transceiver.
`These signals may indicate the direction of the mobile
`transceiver with respect to the base station. This long-term
`correlation matrix estimation has as input, the uplink chan
`nel estimate, denoted by h for the 1" multipath:
`
`65
`
`

`

`US 7,054,664 B2
`
`9
`where j=v-1. Define X=(Q,+yI)+Q, (Q,+y|), Q, I',
`where yo-0 is an estimate of the noise variance at the receiver.
`This can be set to a nominal value depending on the average
`user geometries. Then, the optimal beam former is given by:
`
`where
`
`5
`
`10
`
`15
`
`10
`order to maintain that compliance, beam selection is done
`initially at the time of call set-up. The initial beam that is
`selected may be updated at a very slow rate, because any
`change in beam would require a change in the S-CPICH ID
`phase reference, which may require an RRC signaling
`request. This approach may be referred to as the quasi-static
`beam reference user specific beam forming approach.
`Embodiments of the present invention may provide
`improvements that are transparent to a user who is using a
`mobile transceiver. Additionally, embodiments of the
`present invention may be standards compliant and may not
`require any change in current specifications. Performance
`enhancements when multiple S-CPICHs are allowed to be
`used as phase references by the user equipment are possible,
`although that case may not be strictly compliant with the
`current UMTS standards.
`For channels with Small angle-spread, embodiments of
`the present invention may provide improved performance.
`Specifically, embodiments of the present invention may
`achieve improved beam forming gain for a given number of
`antennas. For larger angle-spreads, performance may
`approach that of a pure fixed beam forming system, which
`may make communication systems more robust to angle
`spread than a system in which the primary common pilot
`channel (P-CPICH) is employed as the phase reference.
`Embodiments of the present invention may be fair, in that
`all users may get similar performance irrespective of their
`geographic location. Additionally, embodiments of the
`present invention may be more robust to delays in signaling
`a change in phase-reference. When a user moves from the
`coverage area of one beam to another, RRC layer signaling
`may be involved to indicate the mobile to change the
`phase-reference to another S-CPICH ID.
`Embodiments of the present invention may avoid scal
`loping losses inherently, without the added complexity and
`cost associated with systems in which many non-orthogonal
`beams per sector are defined or systems that apply beam
`Sweeping (a con