US 7,340,017 B1
`(10) Patent No.:
`a2) United States Patent
`Banerjee
`(45) Date of Patent:
`Mar. 4, 2008
`
`
`US007340017B1
`
`(54) SYSTEM AND METHOD FOR FINGER
`MANAGEMENTIN A RAKE RECEIVER
`
`(75)
`
`Inventor: Debarag N. Banerjee, Sunnyvale, CA
`(US)
`:
`:
`:
`:
`(73) Assignee: National Semiconductor Corporation,
`Santa Clara, CA (US)
`Subject to any disclaimer, the term ofthis
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 899 days.
`(21) Appl. No.: 10/209,650
`
`(*) Notice:
`
`6,625,197 B1*
`6,697,417 B2*
`
`6,731,676 B2*
`7,184,457 B2*
`2002/0006158 AL*
`2002/0045443 Al*
`2002/0094017 Al*
`2003/0072390 A1l*
`2003/0142730 AL*
`2004/0057538 Al*
`
`9/2003 Lundbyet al. ow... 375/130
`2/2004 Fernandez-Corbaton
`et al. cece eeee 375/147
`
`5/2004 Rick et al. eee 375/148
`
`2/2007 Schmidl et al... 375/130
`1/2002 Schmidlet al.
`............. 375/150
`................. 455/421
`4/2002 Hunzinger
`
`7/2002 Wang.esseseeeseeaseesees 375/144
`
`4/2003 Corbaton et al... 375/316
`7/2003 Lin wee eeeeeeee 375/147
`3/2004 Sathiavagcoswaran
`et al.
`ceseeeceesteesseseeeeeee 375/350
`
`* cited by examiner
`Primary Examiner—Jean B. Corrielus
`
`Jul. 30, 2002
`
`(22)
`(51)
`
`(56)
`
`Filed:
`Int. Cl.
`(2006.01)
`HO4B 1/00
`(52) US. Ch.
`cecccccccscevessssssssssssssssssseeeee 375/348; 375/349
`(58) Field of Classification Search
`375/148
`375/147. 205. 150 149. 130. 140 142 67.
`o_— 375/347. 349: 455/421 456
`See application file for com: lete search histo °
`P
`ty:
`PP
`References Cited
`
`(57)
`ABSTRACT
`In a RAKEreceiver capable of detecting and combining a
`plurality of multipath signals, a controller for managing the
`assignmentofthe plurality of multipath signals to fingers of
`the RAKE receiver. The controller determines a phase
`difference between a selected multipath signal and a first
`multipath signal assigned to a first finger of the RAKE
`receiver and does notassign the selected multipath signal to
`a second finger of the RAKE receiver unless the phase
`difference is greater than one-half chip. If the phase differ-
`ence is less that one-half chip, the controller assigns the
`stronger of the selected multipath signal and the first mul-
`U.S. PATENT DOCUMENTS
`tipath signalto the first finger of the RAKE receiver. If the
`SATLA9T A *®
`11/1995 Zehavi occ 375/142
`
`finger power falls below a certain threshold,
`the finger
`5,490,165 A *
`2/1996 Blakeney et al.
`........... 370/335
`
`internal states (viz. channel estimate and delay estimate) are
`6,078,611 A *
`6/2000 La Rosa et al.
`............ 375/147
`
`maintained while the outputof the finger is not processed. If
`. 455/456.2
`..........
`6,249,680 BL*
`6/2001 Wax et al.
`
`
`
`6,269,075 BI* 7/2001 Tran oo...eeecece 370/206 the finger power exceeds the threshold anytime within a
`
`6,320,898 BL* 11/2001 Newsonetal. ............. 375/144
`specified time interval, the normal activities of the finger are
`
`6,345,078 BI™ 2/2002 Basso -v-s-sceesseeeesssen 375/349
`restored. If the power remains lower than the threshold for
`4/2002 Yamashita «0.0.0.0... 375/149
`6,377,614 B1L*
`6/2002 Ito ceecesssseccsssessssseeeesees 375/347
`6,408,039 BL*
`that time period, the finger is deactivated.
`
`3/2003 Regeet al. we. 370/335
`6,532,222 B1*
`6,618,434 B2*
`9/2003 Heidari-Bateni et al.
`... 375/148
`
`27 Claims, 6 Drawing Sheets
`
`411
`
`\
`
`
`
`
`FROM MULTIPATH
`SEARCHER200 —J...|
`
`
`
`5
`=
`FINGER }~ 390
`
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`7
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`tt
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`311
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`331
`332
`333
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`320
`330
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`it
`
`334
`
`1
`
`EXHIBIT 1015
`
`1
`
`EXHIBIT 1015
`
`

`

`U.S. Patent
`
`Mar.4, 2008
`
`Sheet 1 of6
`
`US 7,340,017 B1
`
`TO / FROM PUBLIC
`PHONE SYSTEM
`
`TO/FROM
`INTERNET
`
`140
`
`
`12
`
`---~2
`
`FIG. 1
`
`101
`
`/
`
`y
`
`2
`
`

`

`U.S. Patent
`
`Mar. 4, 2008
`
`Sheet 2 of 6
`
`US 7,340,017 B1
`
`
`
`AYVONNOdAYVONNOG
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`
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`LOTSAWVes
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`vivivlvlvlwlvlvlelvlelele
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`
`3
`
`
`
`

`

`U.S. Patent
`
`Mar. 4, 2008
`
`Sheet 3 of 6
`
`US 7,340,017 B1
`
`YAONIS
`
`LWSOW
`
`YANISWODa||
`
`H1IVdLINWWOYs
`002YAHONVAS
`
`
`
`€‘Ola
`
`4
`
`

`

`U.S. Patent
`
`Mar.4, 2008
`
`Sheet 4 of6
`
`US 7,340,017 B1
`
`
`
`
`
`ASSIGN TO
`MULTIPATH
`NEW BS 2~ES
`
`VERIFICATION
`SEARCH ON|404
`FINGER
`ACTIVE SET
`NO
`
`
`
`COMPARE TARGET SIGNAL
`PHASE TO EACH
`
`ACTIVE FINGER PHASE
`
`
`
`
`IS ABSOLUTE VALUE OF
`
`
`(TARGET PHASE - ACTIVE
`
`
`PHASES) > MINIMUM PHASE ?
`
`402
`
`415
`
`CELL SEARCH DETECTS
`BASE STATIONS
`
`406
`
`
`
`420
`
`425
`
`
`
`
`
`
`IS TARGET POWER>
`coMBINING THRESHOLD ?
`
`430%
`
`
`
`IS TARGET SIGNAL
`POWER> (WEAKEST\ NO
`
`FINGER POWER+
`
`
`
`HYSTERESIS) ?
`
`
`
`
`400
`
`450
`
`DEASSIGN
`WEAKEST
`FINGER
`
`445
`
`460
`
`Aaeee iia aca cer wer wr ere
`
`
`
`5
`
`

`

`U.S. Patent
`
`Mar.4, 2008
`
`Sheet 5 of6
`
`US 7,340,017 B1
`
`905
`
`FROM SRP MODE
`
`510
`
`FOR EACH ASSIGNED FINGER,
`CHECK FINGER POWER
`
`915
`
`IS FINGER POWER LESS
`THAN LOCK THRESHOLD?
`
`
`
`520
`
`
`
`KEEP/ADD FINGER
`TO COMBINING SET
`
`
`
`
`
`
`
`
`
`
`
`
`545
`
`550
`
`
`525
`
`REMOVE FINGER FROM
`COMBINING SET
`
`530
`
`IS FINGER POWER LESS
`THAN FADE THRESHOLD ?
`
`RESET FADE
`COUNTER
`
`
`
`540
`
`INCREMENT
`FADE COUNTER
`
`IS FADE COUNTER =
`MAX. FADE DURATION ?
`
`DEASSIGN FINGER
`
`555
`
`REPEAT FOR ALL
`ACTIVE FINGERS
`
`6
`
`

`

`U.S. Patent
`
`Mar.4, 2008
`
`Sheet 6 of6
`
`US 7,340,017 B1
`
`390
`
`FINGER MANAGEMENT CONTROLLER
`
`SIGNAL SEARCHER
`
`DATA PROCESSOR
`
`MEMORY
`
`FIG. 6
`
`7
`
`

`

`US 7,340,017 B1
`
`1
`SYSTEM AND METHOD FOR FINGER
`MANAGEMENTIN A RAKE RECEIVER
`
`TECHNICAL FIELD OF THE INVENTION
`
`10
`
`25
`
`30
`
`35
`
`40
`
`45
`
`55
`
`Thepresent invention relates generally to wireless receiv-
`ers and, more particularly, to an apparatus and a related
`method in a wireless receiver that performs finger manage-
`ment in a RAKEreceiver.
`
`BACKGROUND OF THE INVENTION
`
`Business and consumers use a wide array of wireless
`devices, including cell phones, wireless local area network
`(LAN) cards, global positioning system (GPS) devices,
`electronic organizers equipped with wireless modems, and
`the like. The increased demand for wireless communication
`devices has created a corresponding demand for technical
`improvements to such devices. Generally speaking, wireless
`system designers attempt to minimize the cost of conven-
`tional radio receivers while improving the performance of
`such devices. Performance improvements include, among
`other things,
`lower power consumption, greater range,
`increased receiver sensitivity, lower bit error rates (BER),
`higher transmission rates, and the like.
`Signal fading due to variations in channel characteristics
`is a majorfactor limiting the performance of modern mobile
`wireless communication systems. To compensate for signal
`fading, many modern code division multiple access
`(CDMA) networks use diversity techniques to transmit
`multiple copies of a signal over a channel to a mobilestation.
`In the mobile station, a RAKE receiver uses multiple base-
`bandcorrelators to individually process several signal mul-
`tipath components. The correlator outputs are then com-
`bined to achieve improved performance.
`A RAKEreceiver comprises L fingers, where each of the
`L fingers contains a basebandcorrelator that processes one
`of the multipath components. A typical spread spectrum
`receiver comprises a code phase acquisition circuit that
`detects multipath components of a transmitted signal and
`assigns (or allocates) each of the strongest multipath com-
`ponent signals to one of the L RAKEfingers.
`However, the channel delays associated with the multi-
`path components are non-stationary. As a result, the multi-
`path components allocated to the RAKE fingers may disap-
`pear as the mobile station (e.g., cell phone) moves and the
`channel delay profile changes. Thus,
`it
`is necessary to
`deassign RAKEfingers once their multipath components are
`lost, to continuously look for new multipath components,
`and to assign the new multipath components to deassigned
`RAKEfingers.
`Asystem for assigning (allocating) and deassigning (deal-
`locating) RAKE fingers is discussed in “Grouped RAKE
`Finger ManagementPrinciple for Wideband CDMA”, B. N.
`Vejlgaard et al., IEEE 2000. However, the apparatus dis-
`closed in the Vejlgaard et al. disclosure only takes finger
`power into account when making assignment decisions. A
`very brief fast fading of a multipath component may cause
`the multipath component to be unnecessarily deassigned
`from a RAKEreceiver finger. When the fade ends after a
`very brief period,
`the recovered multipath component is
`reassigned to the RAKEreceiverfinger again. Also, the prior
`art Vejlgaard et al. reference is wasteful of RAKE receiver
`fingers in that it assigns fingers by groupsofthree that do not
`move independently. This increases the number offingers
`required and also decreases the resolvability of the RAKE
`receiver fingers.
`
`2
`Therefore, there is a need in the art for improved RAKE
`receivers. More particularly, there is a need for improved
`methods and apparatuses for managing the assignment and
`deassignment of fingers in a RAKEreceiver.
`
`SUMMARY OF THE INVENTION
`
`The present invention comprises provides a system and
`method for assignment and de-assignment of RAKE
`receiver fingers using multipath search results and fade
`measurements. The RAKEfingers demodulate spread spec-
`trum signals in a cellular system downlink. The finger
`managementroutine consists of two parts: 1) Search Result
`Processing (SRP) mode and 2) Finger Fade Management
`(FFM) mode. Search Result Processing mode decides
`whether a multipath signal detected by a search routine
`should be assigned to a RAKEreceiver finger. The Finger
`Fade Management mode monitors the energy on each mul-
`tipath to detect whether a multipath signal has been lost.
`In FFM mode, when the energy on a finger goes below a
`threshold the finger is no longer combined in the RAKE. If
`the energy stays below that (or another) threshold for a
`certain time interval, the finger is deassigned. In SRP mode,
`if the search routine detects distinct paths that are above a
`certain threshold,
`the paths are assigned to unassigned
`fingers, if there are any available. If there are no unassigned
`fingers left, the detected path is assigned by replacing the
`weakest finger, if the weakest finger is weaker than the
`detected path by a hysteresis factor. The distinctness of the
`paths is maintained in FFM mode by deassigning paths that
`are less than half a chip apart.
`To address the above-discussed deficiencies of the prior
`art, it is a primary object of the present invention to provide,
`for use in a RAKE receiver capable of detecting and
`combining a plurality of multipath signals, a controller for
`managing the assignmentof the plurality of multipath sig-
`nals to fingers of the RAKE receiver. According to an
`advantageous embodiment of the present
`invention,
`the
`controller determines a phase difference between a selected
`multipath signal and a first multipath signal assigned to a
`first finger of the RAKEreceiver and the controller does not
`assign the selected multipath signal to a secondfinger of the
`RAKEreceiver unless the phase difference is greater than
`one-half chip.
`According to one embodiment of the present invention,
`the controller does not assign the selected multipath signal
`to the second finger of the RAKEreceiver unless the phase
`difference at least one chip.
`According to another embodiment of the present inven-
`tion, the controller, in response to a determination that the
`phase difference is less that one-half chip, assigns the
`stronger of the selected multipath signal and the first mul-
`tipath signal to the first finger of the RAKEreceiver.
`According to still another embodiment of the present
`invention, the controller is further capable of determining if
`a multipath signal is assigned to each finger of the RAKE
`receiver and, in response to a determination that no unas-
`signed fingers are available, the controller determines the
`signal powerofall multipath signals assigned to the fingers
`of the RAKEreceiver and identifies a third finger having the
`weakest multipath signal assigned thereto.
`According to yet another embodiment of the present
`invention, the controller is further capable of determining if
`a signal powerof the selected multipath signal exceeds the
`weakest multipath signal by at least a hysteresis threshold
`value.
`
`8
`
`

`

`US 7,340,017 B1
`
`3
`According to a further embodimentof the present inven-
`tion, the controller, in response to a determination that the
`signal power of the selected multipath signal exceeds the
`weakest multipath signal by at least the hysteresis threshold
`value, assigns the selected multipath signal
`to the third
`finger.
`According to a still further embodiment of the present
`invention, the controller is further capable of determining if
`the signal powerof an assigned multipath signal assigned to
`a fourth finger of the RAKE receiver is less than a fade
`threshold value.
`
`According to a yet further embodiment of the present
`invention, the controller, in response to a determination that
`the signal powerofthe assigned multipath signalis less than
`the fade threshold value, is further capable of determining a
`time duration during which the assigned multipath signals
`has been less than the fade threshold value.
`
`In one embodimentof the present invention, the control-
`ler, in response to a determination that the time duration
`during which the assigned multipath signals has been less
`than the fade threshold value exceeds a maximum fade
`
`duration value, deassigns the assigned multipath signal from
`the fourth finger.
`Before undertaking the DETAILED DESCRIPTION OF
`THE INVENTIONbelow, it may be advantageous to set
`forth definitions of certain words and phrases used through-
`out this patent document: the terms “include” and “com-
`prise,” as well as derivatives thereof, mean inclusion without
`limitation; the term “or,” is inclusive, meaning and/or; the
`phrases “associated with” and “associated therewith,” as
`well as derivatives thereof, may mean to include, be
`included within, interconnect with, contain, be contained
`within, connect to or with, couple to or with, be communi-
`cable with, cooperate with, interleave, juxtapose, be proxi-
`mate to, be boundto or with, have, have a property of, or the
`like; and the term “controller” means any device, system or
`part thereof that controls at least one operation, such a
`device may be implemented in hardware, firmware or soft-
`ware, or some combination of at least two of the same. In
`particular, a controller may comprise a data processor and an
`associated memory that execute one or more functions
`associated with the present invention.It should be noted that
`the functionality associated with any particular controller
`may be centralized or distributed, whether
`locally or
`remotely. Definitions for certain words and phrases are
`provided throughoutthis patent document, those of ordinary
`skill in the art should understand that in many, if not most
`instances, such definitions apply to prior, as well as future
`uses of such defined words and phrases.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`For a more complete understanding of the present inven-
`tion and its advantages, reference is now made to the
`following description taken in conjunction with the accom-
`panying drawings, in which like reference numerals repre-
`sent like parts:
`FIG.1 illustrates an exemplary wireless network in which
`mobile station RAKE receivers using channel estimation
`techniques according to the principles of the invention may
`be used.
`
`FIG. 2 is a timing diagram illustrating the modulation
`pattern for the commonpilot channel (CPICH)signals in the
`wireless network in FIG. 1 according to an exemplary
`embodimentof the present invention;
`
`40
`
`45
`
`50
`
`4
`FIG.3 is a high-level block diagram of a RAKEreceiver
`in an exemplary mobile station according to one embodi-
`ment of the present invention;
`FIG. 4 is a flow diagram illustrating the operation of
`Search Result Processing (SRP) mode according to an
`exemplary embodimentof the present invention;
`FIG. 5 is a flow diagram illustrating the operation of
`Finger Fade Management (FFM) mode according to an
`exemplary embodimentof the present invention; and
`FIG.6 is a diagram illustrating an exemplary embodiment
`of a finger management controller of the present invention.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`FIGS. 1 through 6, discussed below, and the various
`embodiments used to describe the principles of the present
`invention in this patent document are by way ofillustration
`only and should not be construed in any way to limit the
`scope of the invention. Those skilled in the art will under-
`stand that the principles of the present invention may be
`implementedin any suitably arranged mobile station RAKE
`receiver.
`
`in
`FIG. 1 illustrates exemplary wireless network 100,
`which mobile station RAKE receivers using channel esti-
`mation techniques according to the principles of the present
`invention may be used. Wireless network 100 comprises a
`plurality of cell sites 121-123, each containing a basestation
`(BS), such as BS 101, BS 102, or BS 103. Base stations
`101-103 communicate with a plurality of mobile stations
`(MS) 111-114 over, for example, code division multiple
`access (CDMA)channels. Mobile stations 111-114 may be
`any suitable wireless devices, including conventional cellu-
`lar radiotelephones, PCS handset devices, personal digital
`assistants, portable computers, or metering devices. The
`present invention is not limited to mobile devices. Other
`types of access terminals, including fixed wireless terminals,
`may be used. However, for the sake of simplicity, only
`mobile stations are shown and discussed hereafter.
`
`Dotted lines show the approximate boundaries ofthe cell
`sites 121-123 in which base stations 101-103 are located.
`The cell sites are shown approximately circular for the
`purposes ofillustration and explanation only. It should be
`clearly understood that the cell sites may have other irregu-
`lar shapes, depending on the cell configuration selected and
`natural and man-made obstructions.
`As is well known in the art, cell sites 121-123 are
`comprised of a plurality of sectors (not shown), each sector
`being illuminated by a directional antenna coupled to the
`base station. The embodiment of FIG.1 illustrates the base
`station in the center of the cell. Alternate embodiments
`
`position the directional antennas in corners of the sectors.
`The system of the present invention is not limited to any
`particular cell site configuration.
`In one embodimentof the present invention, BS 101, BS
`102, and BS 103 comprise a base station controller (BSC)
`and one or more base transceiver subsystem(s) (BTS). Base
`station controllers and base transceiver subsystems are well
`knownto those skilled in theart. A base station controller is
`
`a device that manages wireless communications resources,
`including the base transceiver stations, for specified cells
`within a wireless communications network. A base trans-
`ceiver subsystem comprises the RF transceivers, antennas,
`and other electrical equipment located in each cellsite.
`BS 101, BS 102 and BS 103 transfer voice and data
`signals between each other and the public switched tele-
`phone network (PSTN) (not shown) and the Internet via
`
`9
`
`

`

`US 7,340,017 B1
`
`5
`communication line 131, mobile switching center (MSC)
`140, and packet data serving node (PDSN) 150. MSC 140 is
`a switching device that provides services and coordination
`between the subscribers in a wireless network and external
`networks, such as the PSTN or Internet.
`In the exemplary wireless network 100, MS 111 is located
`in cell site 121 and is in communication with BS 101. MS
`113 is located in cell site 122 and is in communication with
`BS 102. MS 114 is located in cell site 123 and is in
`communication with BS 103. MS 112 is also located close
`
`to the edge of cell site 123 and is movingin the direction of
`cell site 123, as indicated by the direction arrow proximate
`MS112. At somepoint, as MS 112 movesinto cell site 123
`and out of cell site 121, a hand-off will occur. In an alternate
`embodiment, any of the mobile stations may be in commu-
`nication with a multiplicity of base stations, at least includ-
`ing the base station belongingto the cell it is located in. This
`is known as soft handoff in the art.
`The base stations may transmit from a single antenna or
`from two antennas. If two antennas are used,
`the base
`stations may use transmit diversity (e.g., space-time transmit
`diversity (STTD)) by coding data in a space-time code and
`transmitting the pilot symbols in an orthogonalpattern, such
`as the pattern illustrated in FIG. 2.
`FIG.2 illustrates timing diagram 200, which depicts the
`modulation pattern for the commonpilot channel (CPICH)
`signals in wireless network 100 according to an exemplary
`embodimentofthe present invention. In FIG. 1, each of BS
`101-BS 103 has two antennas that may be used to commu-
`nicate with MS 111-MS 114. Each of base stations 101-103
`
`may use a single antenna to communicate in a non-trans-
`mission diversity (non-TD) mode with the mobile stations.
`However, in an advantageous embodiment of the present
`invention, each of base stations 101-103 may combat the
`effects of multipath fading by transmitting from two anten-
`nas in a space-time transmit diversity (STTD) mode.
`In an exemplary embodiment, wireless network 100 is
`compatible with the 3’ Generation Partnership Project
`(3GPP) standard. In a 3GPP system, during non-TD mode,
`a commonpilot channel (CPICH)signal is transmitted as a
`quadrature signal from a single antenna using the pattern
`shown for Antenna 1 in FIG. 2, where A=14j. During STTD
`mode, a first common pilot channel (CPICH) signal
`is
`transmitted as a first quadrature signal from a first antenna
`using the pattern shown for Antenna 1 in FIG. 2, and a
`second commonpilot channel (CPICH)signalis transmitted
`as a second quadrature signal from a second antenna using
`the pattern shown for Antenna 2 in FIG.2.
`FIG. 3 is a high-level block diagram of RAKE receiver
`300 in exemplary mobile station 111 according to one
`embodimentof the present invention. RAKE receiver com-
`prises antenna 301, radio frequency (RF) front-end block
`305, L fingers, including exemplary fingers 310, 320 and
`330, combiner 340, and finger management controller 390.
`Finger 310 comprises delay element 311, multiplier 312,
`summer 313 and multiplier 314. Finger 320 comprises delay
`element 321, multiplier 322, summer 323 and multiplier
`324. Finger 330 comprises delay element 331, multiplier
`332, summer 333 and multiplier 334.
`RF front-end block 305 downconverts the incoming RF
`signals received from antenna 301 and produces a baseband
`or intermediate frequency signal, which is sampled and
`quantized by an analog-to-digital converter (ADC) to pro-
`duce a sequence of digital values, the signal R. The signal R
`is supplied as the input to each of the L fingers. In each of
`the L fingers,there is a correlator formed by a multiplier and
`a summer. For example,
`in finger 310,
`the correlator is
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`formed by multiplier 312 and summer313, in finger 320, the
`correlator is formed by multiplier 322 and summer 323, and
`in finger 330, the correlator is formed by multiplier 332 and
`summer 333.
`
`In each finger, the signal R is initially delayed by some
`time delay D(n) by the delay elements. The output of each
`delay element is the input of the correlator for that finger.
`Thus,
`the correlators are synchronized to each of the L
`strongest multipath components by delaying the received
`signal R in each finger by an appropriate amount of time
`Dn). The delayed samples of the received signal R are then
`correlated with the chip pattern,c(k), to produce a correlated
`output. The correlated outputs of the correlators are then
`weighted by coeflicients b(n) by the multipliers 314, 324,
`and 334. Combiner 340 combines the weighted outputs and
`the resulting DATA OUTsignalis the final basebandsignal.
`The weighting coefficients b(n) in each of the L fingers of
`RAKEreceiver 300 are calculated by a channel estimation
`filter that uses the pilot channel signals transmitted by base
`stations 101, 102, and 103 and that optimizes the weighting
`coefficients b(n) over a range of Doppler frequencies using
`the average MMSEcriterion. In an exemplary embodiment,
`a digital signal processor (DSP) performs channel estima-
`tion.
`
`invention
`According to the principles of the present
`RAKEreceiver 300 comprises finger management control-
`ler 390, which is used to assign (allocate), deassign (deal-
`locate), activate (combine the output of the finger with the
`outputs of the other fingers) and deactivate (not combine the
`output of the finger with the outputs of the other fingers)
`each of the L fingers to the strongest multipath component
`signals. In an advantageous embodiment, as shown in FIG.
`6, finger management controller 390 may comprise a data
`processor 610 and an associated memory 620 that execute
`one or more finger management functions associated with
`the present
`invention.
`In particular,
`finger management
`controller 390 may comprise a portion of the control soft-
`ware executed by a digital signal processor (DSP). Advan-
`tageously, finger management controller 390 may include or
`be coupled to a signal searcher 630 that can detect and
`measure the strength of multipath signals received from base
`stations.
`
`Finger managementcontroller 390 performs two primary
`functions: 1) Search Result Processing (SRP) mode opera-
`tions and 2) Finger Fade Management (FFM) mode opera-
`tions. In SRP mode, finger management controller 390
`decides whether a multipath signal detected by a search
`routine should be assigned to a RAKEreceiver finger. In
`FFM mode, finger management controller 390 monitors the
`energy of the multipath signal on each assigned finger to
`detect whether a multipath signal has been lost.
`In FFM mode, when the energy on a finger goes below a
`preset threshold value, finger management controller 390
`blocks combiner 340 from combining that finger into the
`DATA OUTsignal(1.e. deactivates that finger). If the energy
`stays below that (or another) threshold value for a certain
`time interval, the finger is deassigned. When a finger is
`deassigned, finger management controller 390 may remove
`powerto theentire finger, thereby reducing power consump-
`tion.
`In SRP mode,if finger managementcontroller 390 deter-
`minesthat certain distinct paths are above a certain thresh-
`old, those paths are assigned to unassignedfingers, if any are
`available. If there are no unassigned fingers left, finger
`management controller 390 assigns the detected path by
`replacing the weakest finger, if the weakest finger is weaker
`than the detected path by a hysteresis factor. According to an
`
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`

`US 7,340,017 B1
`
`7
`finger
`invention,
`exemplary embodiment of the present
`managementcontroller 390 maintains the distinctness of the
`paths in FFM mode bydeassigning pathsthat are less than
`half a chip apart.
`In FIG.3, finger management controller 390 monitors the
`output of each finger by receiving and monitoring the output
`of the final multiplier in each finger (e.g., multipliers 314,
`324 and 334). However, it should be understood that this is
`by way of illustration only and should not be construed so
`as to limit the scope of the present invention. In alternate
`embodiments, finger management controller 390 may deter-
`mine the signal strength in each finger by monitoring, for
`example,
`the unweighted output of the summer in each
`finger (e.g., summers 313, 323 and 333).
`
`Finger Fade Management (FFM) Mode
`FFM mode is executed every frame. The important
`parameters used by finger management controller 390 in
`FFM modeare:
`
`1) Fade Threshold;
`2) Maximum Fade Duration;
`3) Lock Threshold; and
`4) Hysteresis.
`The following description of the present invention sets
`forth particular values for selected parameters and other
`criteria. It should be understood that the particular values
`chosen are by way of example only and should not be
`construed so as to limit the scope of the present invention.
`Those skilled in the art will readily understand how to
`modify the particular values chosen andset forth herein in
`order to adapt
`the present
`invention to other particular
`environments or different configurations.
`Fade Threshold
`The choice of the Fade Threshold and Maximum Fade
`
`Duration parameter values are interrelated. For the purpose
`of fade determination, the Fade Threshold value needs to be
`chosen such that the probability of triggering the fade timer
`given that the mobile is in a temporary fast fade (as opposed
`to a more permanent change in propagation condition) is
`limited. This probability is referred to herein as thefalse loss
`alarm probability, P,. For Rayleigh fading, the probability of
`the signal energy going below the Fade Threshold signal
`level, R, when the local average is Q is:
`
`P=Pr[E,<RIE{E,}=Q\=1-e8?
`
`Choosing this false alarm probability, P,;
`(arbitrarily) yields:
`R/AQ=-23 dB.
`
`[1]
`
`to be 0.005
`
`The P-CPICH channel typically has a value of Ec/Ior of
`about -7 dB, as coded for default parameters. This roughly
`corresponds to an Ec/Io threshold of -30 dB for the Fade
`Threshold level, R (assuming that out-of-cell interference
`and ISI had been minimal). Thus,
`the Fade Threshold
`parameter value for this false alarm probability is set to be
`-30 dB.
`
`Maximum Fade Duration
`
`Given Fade Threshold, R, the average fade duration for
`Rayleigh fading may be obtained as:
`
`t=
`
`eX _ |
`Sn 2n(R/Q)
`
`where,
`R=Fade Threshold level;
`Q=average energy; and
`f,,maximum Doppler frequency.
`The fade counter is designed to check whether the
`decrease in the signal level below the Fade Threshold is due
`to a fast fade or not. A test for such a hypotheses may be
`madebysetting the value of Maximum Fade Duration equal
`to the average fade duration for a threshold R=Fade Thresh-
`old. Since this test needs to work for all typical mobile
`velocities,
`the slowest among the typical channel cases
`needs to be considered. This corresponds to a mobile speed
`of about 3 kmph, and hence a Doppler of about 6 Hz for a
`carrier frequency of 2 GHz.
`Using Equation 2 above, the average fade duration for
`-23 dBis about 4.72 ms, which maybe usedas the value of
`Maximum Fade Duration when the Fade Threshold levelis
`set to -30 dB.
`For other values of false fade alarm probabilities, P,, the
`following may be used as parameter values:
`
`Py
`0.005
`0.01
`0.02
`0.05
`0.1
`0.2
`0.5
`
`FT (dB)
`30.0
`-27.0
`-23.9
`-19.9
`-16.8
`-13.5
`-8.6
`
`MED(ms)
`4.72
`6.70
`9.55
`15.45
`22.76
`35.19
`79.86
`
`Lock Threshold
`
`The Lock Threshold parameter needs to be chosen such
`that whenthe signal from a particular multipath signalfalls
`below that level, its inclusion does not improve the prob-
`ability of error by a significant amount. This can be accom-
`plished by considering the Chernoff bound on the probabil-
`ity of error given by:
`
`L
`
`r.<| |&1
`
`etNEcllo
`1-(N/{G,Np)
`
`where,
`a E,/l,=Ec/Io of the Ith multipath;
`N=Spreading factor;
`G,,=Pilog gain; and
`N,=Integration time for channel estimation.
`If wireless network 100 conform to the 3” Generation
`Partnership Project (3GPP) standard,
`it
`is assumed that
`N=256 and G,=7 dBare typical values. In RAKEreceiver
`300, the pilot channel signalis integrated over 256 chips and
`filtered using a single pole IIRfilter with forgetting factor of
`around 0.9. This yields an effective N,=2560.
`Using these parameters in Equation 3, the probability of
`error actually starts increasing when the multipath SIR
`value, «7 E,/I,, is below -41 dB. Thus, the Lock Threshold
`value should be set above this limit. Although the factor
`becomes less than 1 at -41 dB,
`it starts to significantly
`improvethe link quality only when the SIR is below -30 dB.
`Asthis is also the recommended level for the Fade Thresh-
`old value, the same level of —-30 dB is recommended for the
`Lock Threshold as well. In practice, having the same level
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`US 7,340,017 B1
`
`9
`for the Lock Threshold and the Fade Threshold makes the
`
`computation less by reducing one step in the algorithm.
`
`Hysteresis
`The Hysteresis value is the minimum difference in power
`(in dB) between an existing multipath and the detected
`multipath that must be satisfied in order to perform a finger
`reassignment. There are two sources of error that the hys-
`teresis should guard against: 1) the possibility of a tempo-
`rary fade in the existing path and 2) an error in computing
`the power of the searched multipath.
`Let the Hysteresis value for guarding against temporary
`fade be H,. Then, the probability of false change (i.e., the
`probability that the average energy (Q) of the old multipath
`is less than the energy of the new multipath given that the
`instantaneous energy is less than 02/H,) is (for Rayleigh
`fading):
`
`10
`Hence, the scaling factor should be
`
`E |searcher
`
`z
`Ec |raKkE
`
`2
`_ NSearcher
`2
`Nake
`
`2
`_ (=
`| = 18 dB.
`
`10
`
`