Abstract:
In a RAKE receiver capable of detecting and combining a plurality of multipath signals, a controller for managing the assignment of the 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 not assign 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 difference is less that one-half chip, the controller assigns the stronger of the selected multipath signal and the first multipath signal to the first finger of the RAKE receiver. If the finger power falls below a certain threshold, the finger internal states (viz. channel estimate and delay estimate) are maintained while the output of the finger is not processed. If the finger power exceeds the threshold anytime within a specified time interval, the normal activities of the finger are restored. If the power remains lower than the threshold for that time period, the finger is deactivated.

Description:
TECHNICAL FIELD OF THE INVENTION 
   The present invention relates generally to wireless receivers and, more particularly, to an apparatus and a related method in a wireless receiver that performs finger management in a RAKE receiver. 
   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 conventional 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 major factor 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 mobile station. In the mobile station, a RAKE receiver uses multiple baseband correlators to individually process several signal multipath components. The correlator outputs are then combined to achieve improved performance. 
   A RAKE receiver comprises L fingers, where each of the L fingers contains a baseband correlator 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 component signals to one of the L RAKE fingers. 
   However, the channel delays associated with the multipath components are non-stationary. As a result, the multipath components allocated to the RAKE fingers may disappear as the mobile station (e.g., cell phone) moves and the channel delay profile changes. Thus, it is necessary to deassign RAKE fingers once their multipath components are lost, to continuously look for new multipath components, and to assign the new multipath components to deassigned RAKE fingers. 
   A system for assigning (allocating) and deassigning (deallocating) RAKE fingers is discussed in “Grouped RAKE Finger Management Principle for Wideband CDMA”, B. N. Vejlgaard et al., IEEE 2000. However, the apparatus disclosed 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 RAKE receiver finger. When the fade ends after a very brief period, the recovered multipath component is reassigned to the RAKE receiver finger again. Also, the prior art Vejlgaard et al. reference is wasteful of RAKE receiver fingers in that it assigns fingers by groups of three that do not move independently. This increases the number of fingers required and also decreases the resolvability of the RAKE receiver fingers. 
   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 RAKE receiver. 
   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 RAKE fingers demodulate spread spectrum signals in a cellular system downlink. The finger management routine 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 RAKE receiver finger. The Finger Fade Management mode monitors the energy on each multipath 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 assignment of the plurality of multipath signals 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 RAKE receiver and the controller does not assign the selected multipath signal to a second finger of the RAKE receiver 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 RAKE receiver unless the phase difference at least one chip. 
   According to another embodiment of the present invention, 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 multipath signal to the first finger of the RAKE receiver. 
   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 unassigned fingers are available, the controller determines the signal power of all multipath signals assigned to the fingers of the RAKE receiver 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 power of the selected multipath signal exceeds the weakest multipath signal by at least a hysteresis threshold value. 
   According to a further embodiment of the present invention, 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 power of 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 power of the assigned multipath signal is 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 embodiment of the present invention, the controller, 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 INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” 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 communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to 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 software, 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 throughout this 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 invention and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent 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 common pilot channel (CPICH) signals in the wireless network in  FIG. 1  according to an exemplary embodiment of the present invention; 
       FIG. 3  is a high-level block diagram of a RAKE receiver in an exemplary mobile station according to one embodiment of the present invention; 
       FIG. 4  is a flow diagram illustrating the operation of Search Result Processing (SRP) mode according to an exemplary embodiment of the present invention; 
       FIG. 5  is a flow diagram illustrating the operation of Finger Fade Management (FFM) mode according to an exemplary embodiment of 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 of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged mobile station RAKE receiver. 
     FIG. 1  illustrates exemplary wireless network  100 , in which mobile station RAKE receivers using channel estimation 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 base station (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 cellular 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 of the cell sites  121 - 123  in which base stations  101 - 103  are located. The cell sites are shown approximately circular for the purposes of illustration and explanation only. It should be clearly understood that the cell sites may have other irregular 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 embodiment of 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 known to those skilled in the art. 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 transceiver subsystem comprises the RF transceivers, antennas, and other electrical equipment located in each cell site. 
   BS  101 , BS  102  and BS  103  transfer voice and data signals between each other and the public switched telephone network (PSTN) (not shown) and the Internet via 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 moving in the direction of cell site  123 , as indicated by the direction arrow proximate MS  112 . At some point, as MS  112  moves into 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 communication with a multiplicity of base stations, at least including the base station belonging to 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 orthogonal pattern, such as the pattern illustrated in  FIG. 2 . 
     FIG. 2  illustrates timing diagram  200 , which depicts the modulation pattern for the common pilot channel (CPICH) signals in wireless network  100  according to an exemplary embodiment of the present invention. In  FIG. 1 , each of BS  101 -BS  103  has two antennas that may be used to communicate with MS  111 -MS  114 . Each of base stations  101 - 103  may use a single antenna to communicate in a non-transmission 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 antennas in a space-time transmit diversity (STTD) mode. 
   In an exemplary embodiment, wireless network  100  is compatible with the 3 rd  Generation Partnership Project (3GPP) standard. In a 3GPP system, during non-TD mode, a common pilot 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=1+j. 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 common pilot channel (CPICH) signal is 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 embodiment of the present invention. RAKE receiver comprises 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 produce 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 formed by multiplier  312  and summer  313 , 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 D(n). 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 coefficients b(n) by the multipliers  314 ,  324 , and  334 . Combiner  340  combines the weighted outputs and the resulting DATA OUT signal is the final baseband signal. 
   The weighting coefficients b(n) in each of the L fingers of RAKE receiver  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 MMSE criterion. In an exemplary embodiment, a digital signal processor (DSP) performs channel estimation. 
   According to the principles of the present invention RAKE receiver  300  comprises finger management controller  390 , which is used to assign (allocate), deassign (deallocate), 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 software executed by a digital signal processor (DSP). Advantageously, 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 management controller  390  performs two primary functions: 1) Search Result Processing (SRP) mode operations and 2) Finger Fade Management (FFM) mode operations. In SRP mode, finger management controller  390  decides whether a multipath signal detected by a search routine should be assigned to a RAKE receiver 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 OUT signal (i.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 power to the entire finger, thereby reducing power consumption. 
   In SRP mode, if finger management controller  390  determines that certain distinct paths are above a certain threshold, those paths are assigned to unassigned fingers, 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 exemplary embodiment of the present invention, finger management controller  390  maintains the distinctness of the paths in FFM mode by deassigning paths that 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 determine 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 mode are: 
   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 and set 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 the false loss alarm probability, P f . For Rayleigh fading, the probability of the signal energy going below the Fade Threshold signal level, R, when the local average is Ω is:
 
 P   f   =Pr[E   c   &lt;R|E{E   c }=Ω]=1 −e   −R/Ω   [1]
 
   Choosing this false alarm probability, P f , to be 0.005 (arbitrarily) yields:
 
 R/Ω=− 23 dB.
 
   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   _     =         ⅇ     R   /   Ω       -   1         f   m     ⁢       2   ⁢           ⁢     π   ⁡     (     R   /   Ω     )                       [   2   ]               
where,
 
   R=Fade Threshold level; 
   Ω=average energy; and 
   f m =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 made by setting the value of Maximum Fade Duration equal to the average fade duration for a threshold R=Fade Threshold. 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 dB is about 4.72 ms, which may be used as the value of Maximum Fade Duration when the Fade Threshold level is set to −30 dB. 
   For other values of false fade alarm probabilities, P f , the following may be used as parameter values: 
                                                 P f     FT (dB)   MFD (ms)                                0.005   −30.0   4.72       0.01   −27.0   6.70       0.02   −23.9   9.55       0.05   −19.9   15.45       0.1   −16.8   22.76       0.2   −13.5   35.19       0.5   −8.6   79.86                    
Lock Threshold
 
   The Lock Threshold parameter needs to be chosen such that when the signal from a particular multipath signal falls below that level, its inclusion does not improve the probability of error by a significant amount. This can be accomplished by considering the Chernoff bound on the probability of error given by: 
                   P   e     &lt;       ∏     l   =   1     L     ⁢       ⅇ       -     α   l   2       ⁢       NE   c     /     I   o             1   -     (       N   /     G   p       ⁢     N   p       )                   [   3   ]               
where,
         α l   2  E c /I 0 =Ec/Io of the lth multipath;   N=Spreading factor;   G p =Pilog gain; and   N p =Integration time for channel estimation.       
   If wireless network  100  conform to the 3 rd  Generation Partnership Project (3GPP) standard, it is assumed that N=256 and G p =7 dB are typical values. In RAKE receiver  300 , the pilot channel signal is integrated over 256 chips and filtered using a single pole IIR filter with forgetting factor of around 0.9. This yields an effective N p =2560. 
   Using these parameters in Equation 3, the probability of error actually starts increasing when the multipath SIR value, α l   2  E c /I 0 , 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 improve the link quality only when the SIR is below −30 dB. As this is also the recommended level for the Fade Threshold value, the same level of −30 dB is recommended for the Lock Threshold as well. In practice, having the same level 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 hysteresis should guard against: 1) the possibility of a temporary 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 1 . 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 Ω/H 1 ) is (for Rayleigh fading):
 
 P   f   ≦Pr[E   o   &lt;Ω/H   1   |E{E   o }=Ω]=1− e   −Ω/H     1     Ω =1 −e   −1/H     1     [4]
 
   In order to have P e&lt; 0.5, we need H 1 =1.44=1.6 dB. 
   The other source of error is the error involved in the estimation of the power. In an exemplary embodiment of RAKE receiver  300 , the power estimation in the filter is performed over 1024 chips of coherent integration followed by 25 samples of non-coherent integration. Since the mean of the I and Q components are non-zero, the power estimate is Chi-square (χ 2 ) distributed with degree n−1, where n=25, as follows: 
                       (     n   -   1     )     ⁢     E   ^       E     ~       χ   2     ⁡     (     n   -   1     )               [   5   ]               
So, for a given one-sided confidence level
 
                   Pr   ⁡     [     E   &gt;         (     n   -   1     )     ⁢     E   ^           χ     1   -   δ     2     ⁡     (     n   -   1     )           ]       =   δ           [   6   ]               
the noise threshold may be set to
 
                   H   2     =           χ     1   -   δ     2     ⁡     (     n   -   1     )         (     n   -   1     )       .             [   7   ]               
For δ=0.9 and n=25, this yields H 2 =1.38=1.4 dB.
 
   Combining the noise and the fading thresholds yields a total hysteresis threshold of:
 
 H=H   1   +H   2 =3 dB
 
Power Scaling Factor
 
   The finger power estimate needs to be scaled up by a scaling factor in order to make direct comparisons with the searcher power estimates. In both the searcher and the finger, the coherent integration is long enough such that the noise component in the power estimate can be neglected (otherwise the scaling factor depend on the signal-to-noise ratio (SNR)). The power estimate in either case is approximately:
 
 Ê=N   2   E   c   +NI   0   ≅N   2   E   c   [8]
 
Hence, the scaling factor should be
 
   
     
       
         
           
             
               
                 
                   
                     
                       
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                       RAKE 
                     
                   
                 
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   Note that when the searcher power is compared to the finger powers, both the scaling and the hysteresis need to be considered. Hence, a total gain of 21 dB needs to be applied to the finger power estimate. 
   Search Result Processing (SRP) 
   Finger management controller  390  executes SRP mode once every search interval (i.e., every time a search is completed over the entire window of all base station in the active set, modulo a frame). In SRP mode, the important parameters/heuristics used by finger management controller  390  are: 
   1) Combining Threshold; and 
   2) Minimum Phase. 
   Combining Threshold 
   If the energy of a searched multipath is greater than the Combining Threshold, the multipath is considered for finger assignment, subject to subsequent constraints (e.g., finger availability). This is the same as detection of a multipath signal. Hence, this threshold should be equal to the detection threshold in the multipath search circuitry. 
   Minimum Phase 
   The Minimum Phase gives the minimum separation in phase that is allowed between any two finger assignments. Two different techniques are considered: 
   1) Minimum Phase&gt;½ chip—In this scenario, finger management controller  390  only considers paths that are more than ½ chip apart. If two paths are less than or equal to ½ chip apart, the stronger one is assigned. In an advantageous embodiment of the present invention, finger management controller may use a chip spacing of at least one chip. The advantage of this method is that finger assignments are more robust as the unresolvable side lobes typically get eliminated. 
   2) Minimum Phase=0 chips—In this scenario, finger management controller  390  assigns all the paths that are detected to fingers regardless of the separation between paths. If finger management controller  390  employs a delay-locked loop (DLL) that brings the two paths together, then the weaker one is dropped. The problem with this method is that a typical DLL only move the path by maximum of ⅛ chip in each frame. Thus, it takes at least 4 frames in order to drop any spurious multipaths. This leads to noise enhancements for short periods of time as well as excess finger activity (hence, excess power). Also, since the number of fingers is limited, if all fingers get assigned to actual multipaths and their sidelobes, other weaker multipaths cannot be accommodated until the DLLs of finger management controller  390  resolve the ambiguity. 
     FIG. 4  illustrates flow diagram  400 , which depicts the operation of Search Result Processing (SRP) mode according to an exemplary embodiment of the present invention. Initially, the mobile station performs a cell search and detects a new base station (BS) signal (process step  402 ). Next, the mobile station determines if the detected base station signal is coming from a new base station (process step  404 ). If it is a new base station (i.e., Yes), the new base station signal is assigned to a verification finger (process step  406 ) and the BCH CRC value is checked. 
   If the CRC value is invalid, the new base station signal is rejected (process step  490 ). If the CRC value is valid, finger management controller  390  conducts a multipath search (process step  410 ) and compares the phase of the new detected signal (referred to hereafter as the “target signal”) to the phases of the active finger in RAKE receiver  300  (process step  420 ). Alternatively, finger management controller  390  may enter process step  420  when performing a multipath search on the active set of fingers (process step  415 ). In this case, the signal from each finger is treated as if it is a new detected signal and also is referred to hereafter as the “target signal” in step  420  and subsequent steps. 
   Finger management controller  390  determines if the difference between the phase of the target signal and the phase of each one of the active fingers is greater than the Minimum Phase value (process step  425 ). If not, the target signal is rejected (process step  490 ). If the phase difference is greater that the Minimum Phase value, finger management controller  390  determines if the power of the target signal is greater than the Combining Threshold value (process step  430 ). If not, the target signal is rejected (process step  490 ). 
   If the target signal power is greater than the Combining Threshold value, finger management controller  390  determines if a finger is available (process step  435 ). If a finger is available, finger management controller  390  assigns the target signal to an available finger (process step  450 ) and enters finger fade management mode (process step  460 ). If no finger is available, finger management controller  390  determines if the power of the target signal is greater than the sum of the power of the weakest signal on an active finger plus and the Hysteresis value. 
   If the target signal power is not greater than the sum of the weakest signal power plus the Hysteresis value, finger management controller  390  rejects the target signal (process step  490 ). If the target signal power is greater than this sum, finger management controller deassigns the weakest signal (process step  445 ), assigns the target signal to the newly available finger (process step  450 ) and enters finger fade management mode (process step  460 ). 
     FIG. 5  illustrates flow diagram  500 , which depicts the operation of Finger Fade Management (FFM) mode according to an exemplary embodiment of the present invention. FFM mode begins when finger management controller  390  exits Search Result Processing (SRP) mode (process step  505 ). Initially, finger management controller  390  checks the power of the received signal in each active finger (process step  510 ). It is notes that process step  510  and the remaining process steps in flow diagram  500  are repeated for each active finger. 
   For each finger, finger management controller  390  determines if the finger power is less than the Lock Threshold value (process step  515 ). If not, finger management controller  390  either keeps (i.e., for existing finger) the subject finger in, or adds (i.e., for new finger) the subject finger to, the combining set of active fingers combined by combiner  340 . Finger management controller  390  then proceeds to the next subject finger until all fingers are done (process step  555 ). If the finger power is less than the Lock Threshold value, finger management controller  390  removes the subject finger from the combining set of active fingers combined by combiner  340  (process step  525 ). 
   Next, finger management controller  390  determines if the finger power is less than the Fade Threshold value (process step  530 ). If not, finger management controller  390  resets the Fade Counter (process step  535 ) and activates the finger if it was deactivated. Finger management controller  390  then proceeds to the next subject finger until all fingers are done (process step  555 ). However, if the finger power is less than the Fade value, finger management controller  390  increments the Fade Counter value (process step  540 ) and deactivates the finger. 
   In this context, deactivation of a finger implies that the finger continues to perform all signal processing functions (including, for example, channel estimation, delay adjustment, and the like) that are required to maintain the state of the finger, except that the finger output will not be combined (and optionally will not be used in automatic frequency correction). Activation implies that all the normal functions of a RAKE finger (including combining of its output and use in AFC) are restored. 
   It is noted that the channel gain and delay elements stay up-to-date when a finger is activated following deactivation, thereby ensuring better bit error performance. When a finger is freshly assigned, however, these parameters need to be estimated from scratch, thereby degrading performance. 
   Next, finger management controller  390  determines if the Fade Counter value is equal to the Maximum Fade Duration value (process step  545 ). If not, finger management controller  390  proceeds to the next subject finger until all fingers are done (process step  555 ). However, if the Fade Counter value is equal to the Maximum Fade Duration value, finger management controller  390  deassigns the subject finger (process step  550 ). 
   Finger management controller  390  repeats the Finger Fade Management loop until all active fingers are done (process step  555 ). When all active fingers are done, finger management controller  390  returns to SRP mode (process step  560 ). 
   The prior art disclosed in the Vejlgaard et al. reference only takes searcher and finger power into account when making assignment decisions. The present invention uses searched power, finger power measurements, as well as a fade timer in making finger assignment and deassignment decisions. In addition, by activating and deactivating the finger, the present invention enables faster response to fades and has better bit error performance. Also, the prior art is wasteful of RAKE receiver fingers in that it assigns fingers by groups of three that do not move independently. The present invention minimizes the number of fingers required and also preserves the resolvability of the fingers. 
   Although the present invention has been described with several embodiments, various changes and modifications may be suggested to one skilled in the art. It is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.