Patent Publication Number: US-7711041-B2

Title: Signal-to-interference ratio estimation

Description:
CLAIM OF PRIORITY 
   A claim of priority is made to Korean Patent Application No. 2004-84729, filed on Oct. 22, 2004, the contents of which are herein incorporated by reference in their entirety for all purposes. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   Example embodiments of present invention generally relate to a power control adapted for use in a direct sequence code division multiple access (DS-CDMA) system. More particularly, example embodiments of the present invention relates to a signal-to-interference ratio (SIR) estimation adapted for use in a closed-loop power control. 
   2. Description of the Related Art 
   In general, a direct sequence code division multiple access (DS-CDMA) wireless protocol specifies interfaces in wireless communication systems such as a code division multiple access (CDMA) system and a wide-band code division multiple access (W-CDMA) system. In these systems, an accurate power control may be necessary to effectively utilize frequency resources that are commonly shared by a mobile station and a base station according to spread-spectrum characteristics of the wireless communication system. 
   A system capability, possibly larger in a spread spectrum type system, may be increased when the power is optimally controlled, because the system capability of the CDMA system or the W-CDMA system may be highly correlated with an amount of interference signals received on the system, which may be determined by an intensity of a signal transmitted to/from each mobile device. 
   The W-CDMA standard is embodied in the 3rd Generation Partnership Project (3GPP) standard. For example, in the first or second generation system, a digital cellular or a personal communication system, data traffic on a reverse link may be substantially equal to data traffic on a forward link, because the first and second generation communication systems were designed to mainly carry speech and low bit-rate data. 
   However, the third generation communication systems such as W-CDMA are designed to carry not only speech but also packet data having a higher data rate. The third generation communication systems are also adapted to carry higher data-rate wireless Internet services. 
   One example characteristic of the wireless Internet service is that a forward link may provide a service of downloading a large capacity picture movie, voice data, picture data, and program data. The forward link may have more traffic than a reverse link. Therefore, the traffic on the two links may be asymmetric. In the W-CDMA system, capacity limitation of a mobile device may be more likely to be generated at the forward link rather than the reverse link. 
   To overcome the capacity limitation, a closed loop power control may be employed in the forward link of the W-CDMA system in the 3GPP standard to efficiently manage resources of the forward link. 
   The forward closed-loop power control, and those especially based on a signal-to-interference ratio (SIR), may adjust a transmit power of a base station on a forward link based on a received power estimate at a mobile device such that the SIR of each respective mobile device may be constantly maintained. In other words, the transmit power of the base station may be adjusted according to the state of the mobile device, for example by taking into account a near-far effect. Therefore, when the mobile device is not operated (e.g., the mobile device is not connected) but relatively close to a base station or has a relatively low levels of multi-path fading, shadowing, interference from other base stations, etc., transmit power may be reduced. On the other hand, when the mobile device is relatively farther away from the base station, which may create higher bit error rate or a signal reception by the mobile device is poor, the transmit power is increased. 
   In the closed-loop power control based on the SIR estimation, a reference SIR is compared with an SIR estimate per a predetermined power control period (e.g., 1/1500 sec, in the 3GPP). Therefore, the accuracy of the SIR estimation may greatly affect the stability and accuracy of the transmit power control. 
   A conventional SIR estimation method in accordance with an NTT DoCoMo wireless protocol is described in “SIR-Based Transmit Power Control of Reverse Link for Coherent DS-CDMA Mobile Radio,” IEICE Transactions on Communications, July 1998. According to the above disclosed SIR estimation method, after a tentative decision based on a channel estimation using a pilot symbol, the SIR is estimated using an average power per slot and a mean squared error of a dedicated physical channel (DPCH). The tentative decision herein is made such that a hard decision is made for a phase of a channel compensated received signal to return a phase of a non-channel compensated signal. The tentative decision is used for signal power estimation per slot, using not only a pilot symbol of which a phase is known, but also a random data symbol. 
   A transmitting signal including the DPCH signal and a common physical channel (CPICH) signal that are coded and multiplexed to be transmitted in parallel may be represented by equation 1, and a received signal passing through a fading channel and Additive White Gaussian Noise (AWGN) may be represented by the following equation 2: 
                   S   ⁡     (   t   )       =       ∑     i   =     -   ∞       ∞     ⁢     [     {         E     c   ,   cp         ·     G   p     ·     (         C   I     ⁡     (   t   )       +     j   ⁢           ⁢         C   Q     ⁡     (   t   )       ·     (       W     CP   ⁢        i        ⁢   N1     I     +     W     CP   ⁢        i        ⁢   N1     Q       )         +         E     c   ,   dp         ·     G   p     ·     (         D   I     ⁡     (   t   )       +     j   ⁢           ⁢         D   Q     ⁡     (   t   )       ·     (       W     DP   ⁢        i        ⁢   N2     I     +     W     DP   ⁢        i        ⁢   N2     Q       )           }     ·     (       S          i        ⁢   M     I     +     S          i        ⁢   M     Q       )                           [     Equation   ⁢           ⁢   1     ]                   r   ~     ⁡     (   t   )       =       ∑     i   =     -   ∞       ∞     ⁢     [       a   ⁡     (   t   )       ·           [         {         E     c   ,   cp         ·     G   p     ·     (         C   I     ⁡     (   t   )       +     j   ⁢           ⁢         C   Q     ⁡     (   t   )       ·     (       W     CP   ⁢        i        ⁢   N1     I     +     W     CP   ⁢        i        ⁢   N1     Q       )         +         E     c   ,   dp         ·     G   p     ·     (         D   I     ⁡     (   t   )       +     j   ⁢           ⁢       D   Q     ⁡     (   t   )           )     ·     (       W     DP   ⁢        i        ⁢   N2     I     +     W     DP   ⁢        i        ⁢   N2     Q       )         }     ·     (       S          i        ⁢   M     I     +     S          i        ⁢   M     Q       )       ]     ·     ⅇ     jθ   ⁡     (   t   )           +       n   ^     ⁢        i            ]                     [     Equation   ⁢           ⁢   2     ]               
Herein,
     Ec,cp, Ec,dp: average chip energy of the CPICH and the DPCH   N1, N2: spreading factor (SF) of the CPICH and the DPCH   M: length of a scrambling code   G p : channel gain of the CPICH   W I CP|i| N1 +W Q CP|i| N1 , W I DP|i| N2 +W Q DP|i| N2 : orthogonal variable spreading factor (OVSF) of the CPICH and DPCH   S I |i| M +S Q |i| M : scrambling code   {circumflex over (n)}|i|: AWGN   α(t), θ(t): amplitude and phase of the received signal due to the fading   
   After performing descrambling and dispreading processes, respective signals of the DPCH and CPICH that are on a chip basis are converted to signals ZDPCH and ZCPICH that are on a symbol basis, which are represented by equations 3 and 4: 
                   Z   DPCH     =         1     N   2       ⁢       ∑     k   =   0         N   2     -   1       ⁢           ⁢     [       a   ⁡     (   k   )       ·     [         E     c   ,   cp         ·     (         D   I     ⁡     (   k   )       +     j   ⁢           ⁢       D   Q     ⁡     (   k   )           )       ]     ·     ⅇ     jθ   ⁡     (   k   )           ]         +       1     N   2       ⁢       ∑     k   =   0         N   2     -   1       ⁢           ⁢     W     dp   ⁢        k                          [     Equation   ⁢           ⁢   3     ]                 Z   CPICH     =         1     N   1       ⁢       ∑     k   =   0         N   1     -   1       ⁢           ⁢     [       a   ⁡     (   k   )       ·     [         E     c   ,   cp         ·     (         C   I     ⁡     (   k   )       +     j   ⁢           ⁢       C   Q     ⁡     (   k   )           )       ]     ·     ⅇ     jθ   ⁡     (   k   )           ]         +       1     N   1       ⁢       ∑     k   =   0         N   1     -   1       ⁢           ⁢     W     cp   ⁢        k                          [     Equation   ⁢           ⁢   4     ]               
Herein,
   W   DP   |i |=( n   I   |i|+n   Q   |i |)·( S   I   |i|   M   +S   Q   |i|   M )*·( W   I   DP   |i|   N2   +W   Q   DP   |i|   N2 )*   W   CP   |i |=( n   I   |i|+n   Q   |i |)·( S   I   |i|   M   +S   Q   |i|   M )*·( W   I   DP   |i|   N1   +W   Q   DP   |i|   N1 )* 
   After a tentative decision is made for the DPCH symbol, the DPCH symbol is rewritten as equation 5 and the signal power at the time of the SIR estimation is represented as the average power of the DPCH symbol per slot, for which the tentative decision is made as in equation 6: 
                     Z   )     DPCH     =           E     c   ,   dp         ·     1     N   2         +     j   ⁡     [           E     c   ,   dp         ·     1     N   2         ⁢       ∑     k   =   0         N   2     -   1       ⁢           ⁢     [         a   ⁡     (   k   )       ·   sin     ⁢           ⁢     θ   ⁡     (   k   )         ]         ]       +       1       N   2     ⁢               ⁢       ∑     k   =   0         N   2     -   1       ⁢           ⁢     Z        k        I         +     Z        k        Q               [     Equation   ⁢           ⁢   5     ]                   E   ⁡     [       Z   )     DPCH     ]       slot   2     =         E     c   ,   dp       ·     [         (       1     S   2       ⁢     1     N   2       ⁢       ∑     s   =   0         S   2     -   1       ⁢           ⁢       ∑     k   =   0         N   2     -   1       ⁢           ⁢     [         a   ⁡     (     s   ,   k     )       ·   cos     ⁢           ⁢     θ   ⁡     (     s   ,   k     )         ]           )     2     +       (       1     S   2       ⁢     1     N   2       ⁢       ∑     s   =   0         S   2     -   1       ⁢           ⁢       ∑     k   =   0         N   2     -   1       ⁢           ⁢     [     a   ⁢       (     s   ,   k     )     ·   cos     ⁢           ⁢     θ   ⁡     (     s   ,   k     )         ]           )     2       ]       =       E     c   ,   dp       ·     K   DPCH                 [     Equation   ⁢           ⁢   6     ]               
Herein,
   Z |k|=Z   I   |k|+Z   Q   |k|=w|k|· ( D   I ( K )+ jD   Q ( K ))*     S 2 : a number of DPCH symbols per slot   average AWGN per slot:   
   
     
       
         
           
             
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   It can be determined from equation 6 that the signal power is affected by the fading with a signal power attenuation ratio KDPCH. The signal power affected by the fading with the signal power attenuation ratio KDPCH can be experimentally identified. 
     FIG. 1  is a graph illustrating a power attenuation of a signal on a dedicated physical channel (DPCH) that undergoes fading. 
   Referring to  FIG. 1 , as the fading becomes faster, the power attenuation of a signal becomes greater. Compared with an ideal case where there is no fading ( 101 ), the signal power attenuation is generated when a mobile device is moving at a speed of about 120 km/h ( 103 ) and when a mobile device is moving at a speed of about 250 km/h ( 105 ). 
   When energy per symbol to interference power density ratio (Es/No) is low, the AWGN may not approach zero, which may generate an error between an actual signal power attenuation and a calculated value by the signal power attenuation ratio KDPCH. When the energy per symbol to interference power density ratio (Es/No) is relatively high, the AWGN may approach zero so that the signal power attenuation is nearly equal to the signal power attenuation ratio KDPCH. 
   Therefore, in the conventional SIR estimation method, the SIR estimation may not be sufficiently accurate due to the signal power attenuation of a received signal in the fading environment, especially in a fast fading environment. Accordingly, accurate power control depending on a state of a wireless mobile channel may be difficult. 
   SUMMARY OF THE INVENTION 
   In an embodiment of the present invention, a method of estimating a signal-to-interference ratio (SIR) includes estimating a first average channel power per slot of a first channel that is under a fading environment, estimating a second average channel power per slot of a second channel that is under a fading environment substantially the same as the fading environment of the first channel, calculating a second signal power attenuation ratio of the second channel using the second average channel power per slot of the second channel, and calculating a third average channel power per slot of the first channel using the first average channel power per slot estimate and a reciprocal of the second signal power attenuation ratio of the second channel. 
   In another embodiment of the present invention, a method of estimating a signal-to-interference ratio (SIR) includes estimating a first average channel power per slot of a first channel that is under a fading environment, estimating a second average channel power per slot of a second channel that is under a fading environment substantially the same as the fading environment of the first channel, calculating a second signal power attenuation ratio of the second channel using the second average channel power per slot of the second channel, calculating a third average channel power per slot of the first channel using a result of a multiplication of the first average channel power per slot estimate and a reciprocal of the second signal power attenuation ratio of the second channel, estimating a signal-to-interference ratio (SIR) using the third average channel power per slot of the first channel that compensates for the signal power attenuation of the first channel, and comparing the estimated signal-to-interference ratio (SIR) with a reference signal-to-interference ratio (SIR) to generate a power control signal according to a comparison result. 
   Also in another embodiment of the present invention, an apparatus to estimate a signal-to-interference ratio (SIR), the apparatus includes a first signal power estimation unit configured to estimate a first average signal power per slot of a first channel, a second channel signal power estimation unit configured to estimate a signal power attenuation ratio of a second channel, a signal power compensation unit configured to output a compensated third average channel power per slot of the first channel by multiplying a reciprocal of the signal power attenuation ratio of the second channel by the first average channel power per slot of the first channel, an interference power estimation unit configured to estimate an interference power of the first channel, and a signal-to-interference calculation unit configured to calculate the signal-to-interference ratio (SIR) by dividing the compensated third average channel power per slot that are summed for respective paths with the interference power of the first channel that are summed for respective paths. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Example embodiments of the present invention will become more apparent with the following description with reference to the attached drawings. In the drawings like elements are represented by like reference numerals. The drawings are given by way of illustration only and thus do not limit the example embodiments of the present invention. 
       FIG. 1  is a graph illustrating a power attenuation of a signal on a dedicated physical channel (DPCH) that undergoes fading. 
       FIG. 2  is a block diagram illustrating a method of measuring a signal-to-interference ratio (SIR) according to an example embodiment of the present invention. 
       FIG. 3  is a block diagram illustrating an apparatus to measure an SIR according to an example embodiment of the present invention. 
       FIG. 4  is a graph illustrating power on a dedicated physical channel (DPCH) according to an example embodiment of the present invention. 
       FIGS. 5A and 5B  are graphs illustrating measured SIR results for a conventional SIR estimation method and for an example embodiment of the present invention, respectively. 
       FIGS. 6A and 6B  are graphs illustrating SIR jitter characteristics for a conventional SIR estimation method and for an example embodiment of the present invention, respectively. 
       FIG. 7  illustrates conditions of various parameters under which simulations results for  FIGS. 4 ,  5 B and  6 B are generated. 
   

   DESCRIPTION OF THE EXAMPLE EMBODIMENTS 
   Hereinafter, example embodiments of the present invention will be explained in detail with reference to the accompanying drawings. 
   It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements are not limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the example embodiments of the present invention. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
   It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (i.e., “between” versus “directly between”, “adjacent” versus “directly adjacent”, etc.). 
   The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “includes” and/or “including”, when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
   Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
   In a W-CDMA standard, a first channel for which a signal-to-interference ratio (SIR) is to be estimated may be a dedicated physical channel (DPCH). In addition, a second channel that is used to compensate signal power attenuation at the first channel may be a common pilot channel (CPICH). 
   When the CDMA system is based on a communication protocol such as IS-95, the first channel may be a traffic channel and the second channel may be a pilot channel. 
   In one example embodiment of the present invention, a direct sequence code division multiple access (DS-CDMA) standard may be based on a W-CDMA standard. A signal power attenuation of the DPCH caused by fading may be compensated by a reciprocal of a signal power attenuation ratio of, for example, the CPICH. 
   The signal attenuation ratio of the CPICH, which is influenced by the fading along with the DPCH, may be estimated to compensate for the signal power attenuation of the DPCH signal. 
   The CPICH has a signal power attenuation ratio substantially equal to the signal power attenuation ratio of the DPCH, because both the DPCH and the CPICH channel are under the same fading environment. Therefore, the signal power attenuation of the CPICH may be compensated by using the reciprocal of the signal power attenuation ratio of the CPICH in calculating the power of a signal. 
   An average power of a CPICH symbol ZCPICH per slot may be expressed by equation 7, using equation 4: 
   
     
       
         
           
             
               
                 
                   
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   Since the signal power of the CPICH may be affected by substantially the same fading as the DPCH, the signal power attenuation ratio KDPCH of the DPCH may be equal to the signal power attenuation ratio KCPICH of the CPICH, which is identified by equation 8: 
   
     
       
         
           
             
               
                 
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                                       , 
                                       k 
                                     
                                     ) 
                                   
                                 
                               
                               ) 
                             
                             2 
                           
                           + 
                           
                             
                               ( 
                               
                                 
                                   
                                     
                                       a 
                                       _ 
                                     
                                     ⁡ 
                                     
                                       ( 
                                       
                                         s 
                                         , 
                                         k 
                                       
                                       ) 
                                     
                                   
                                   · 
                                   sin 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   
                                     θ 
                                     _ 
                                   
                                   ⁡ 
                                   
                                     ( 
                                     
                                       s 
                                       , 
                                       k 
                                     
                                     ) 
                                   
                                 
                               
                               ) 
                             
                             2 
                           
                         
                         ] 
                       
                       = 
                       
                         K 
                         DPCH 
                       
                     
                   
                 
               
             
             
               
                 [ 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   8 
                 
                 ] 
               
             
           
         
       
     
   
   Generally, a signal power of the CPICH (a pilot channel) may be greater than the signal power of the DPCH. For example, in a 3GPP system, the signal energy of the CPICH may be greater than the signal energy of the DPCH by about 7 dB so that the CPICH has more channel gain compared to the DPCH. When a difference in channel gain between the CPICH and the DPCH is defined as Gp, the effect of the difference in channel gain Gp requires elimination to compensate for the power attenuation in the DPCH by using the signal power attenuation ratio of the CPICH. 
   Therefore, the average power per slot of a square of the CPICH symbol ZCPICH and equation 7 may be used to remove the effect of the Gp. The average power per slot for the square of the CPICH symbol ZCPICH may be calculated as per equation 9: 
                     E   ⁡     [       Z     CPICH   )       2     ]       slot     =       E     c   ,   cp       ·     G   p   2     ·       N   0       N   1                 [     Equation   ⁢           ⁢   9     ]               
Herein, N 0  denotes noise variance.
 
   It may be determined from equation 9 that the average power per slot for the square of the CPICH symbol ZCPICH is not interfered by fading, but rather influenced by AWGN (N0/N1). 
   A ratio of the average power per slot of the square of the CPICH symbol in equation 9 to the average power per slot of the CPICH in equation 7 may be expressed as equation 10: 
                     δ   ^     ⁡     (     l   ,   k     )       =           E   ⁡     [       (     Z   CPICH     )     2     ]       slot         E   ⁡     [     Z   CPICH     ]       slot   2       =       1     K   CPICH       +       1     K   CPICH       ·       N   0       N   1                     [     Equation   ⁢           ⁢   10     ]               
Because the effect of the AWGN is reduced by a spreading factor N1 of the CPICH and the channel gain Gp, N0/N1 may approach zero so that
 
                 δ   ^     ⁡     (     l   ,   k     )       ≅     1     K   CPICH         ,         
and using equation 6, the average power per slot in the DPCH symbol slot may be calculated as per equation 11:
 
   
     
       
         
           
             
               
                 
                   
                     E 
                     ⁡ 
                     
                       [ 
                       
                         
                           Z 
                           ) 
                         
                         DPCH 
                       
                       ] 
                     
                   
                   slot 
                   2 
                 
                 = 
                 
                   
                     
                       E 
                       
                         c 
                         , 
                         cp 
                       
                     
                     · 
                     
                       K 
                       DPCH 
                     
                     · 
                     
                       1 
                       
                         K 
                         CPICH 
                       
                     
                   
                   = 
                   
                     E 
                     
                       c 
                       , 
                       cp 
                     
                   
                 
               
             
             
               
                 [ 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   11 
                 
                 ] 
               
             
           
         
       
     
   
   Namely, as it may be determined by equation 11, the signal power of the DPCH may be multiplied by the reciprocal 1/KCPICH of the signal power attenuation ratio LCPICH due to the fading on the CPICH to compensate for the signal power attenuation of the DPCH. 
     FIG. 2  is a block diagram illustrating a method of measuring a signal-to-interference ratio (SIR) according to an example embodiment of the present invention. 
   Referring to  FIG. 2 , a channel estimator  203  may receive a despread CPICH signal  201  to output a channel estimate  204 . The channel estimate  204  and a despread DPCH signal  202  are complexes which may be multiplied in a multiplier  205  so that compensation is performed for a phase error due to a fading channel. After compensating for phase errors of the channel estimate  204  and the despread DPCH signal  202  for each respective path, a DPCH data symbol  207  may be rake combine, and a tentative decision  206  is performed on the DPCH data symbol  207 . The DPCH data symbol  207  on which the tentative decision has been performed is applied to a first calculating unit  210 . 
   The first calculating unit  210  may include a power calculator and a squarer to calculate an average power of a DPCH slot as per equation 6. 
   A result of the first calculating unit  210  may be output to a filter unit  215 . The filter unit  215  may include a low pass filter (not shown) that is implemented as a first order linear filter of an infinite impulse response (IIR) type. The filter unit  215  may be used to stabilize the effect of fading to improve jitter characteristics of SIR. Therefore, the effect of the fading at the time the SIR estimation is performed should be taken into account when designing the filter unit  215 . 
   When a filter coefficient α is less than 0.5, the fading at a previous time may have a greater influence than the fading at the time the SIR estimation is performed so that the fading associated with the SIR estimation may not be accurately reflected. Therefore, in one example embodiment, the filter coefficient α may be set as 0.7 to reflect the fading at the time the SIR estimation is performed. It is noted that the filter coefficient α may be varied to optimize for each communication system. 
   When an nth symbol of a kth slot in an lth path that is multiplied with an OVSF code of the DPCH is defined as dl(n,k) and the nth symbol of the kth slot in an lth path that is multiplied with the OVSF code of the CPICH is defined as cl(n,k), the average power per slot of the DPCH signal passing through the filter unit  215  may be expressed by equation 12: 
                       S   )       dp   ,   l       ⁡     (   k   )       =         (     1   -   α     )     ·         S   ~       dp   ,   l       ⁡     (     k   -   1     )         +     α   ·              1     S   2       ⁡     [         ∑     n   =   0       N   p       ⁢           ⁢           d   ^     l     ⁡     (     n   ,   k     )       ⁢     ⅇ     -     j   ⁡     (     π   /   4     )               +       ∑     n   =     N   p           S   2     -   1       ⁢           ⁢           d   ^     l     ⁡     (     n   ,   k     )       ⁢     ⅇ       -   j     ⁢           ⁢       ϕ   ^     ⁡     (     n   ,   k     )                 ]            2                 [     Equation   ⁢           ⁢   12     ]               
Herein,
     {circumflex over (φ)}(n,k): a phase for the tentative decision   {circumflex over (d)} l (n,k): a DPCH symbol that is channel compensated for using the CPICH channel estimation   
   Np: a number of pilot symbols in a slot of the DPCH channel 
   The phase for the tentative decision {circumflex over (φ)}(n,k) and DPCH symbol compensated using the CPICH channel estimation {circumflex over (d)} 1 (n,k) may be respectively expressed by equations 13 and 14: 
                     ϕ   ~     ⁡     (     n   ,   k     )       =     max   ⁡     [       ϕ   ∈     {         m   π     /   2     ;     m   =     0   -   3         }       ,       Re   ⁡     [             d   ^     l     ⁡     (     n   ,   k     )       ′     ⁢     ⅇ       -   j     ⁢           ⁢   ϕ         ]       +     π   4         ]               [     Equation   ⁢           ⁢   13     ]                       d   ^     l     ⁡     (     n   ,   k     )       =       ∑     l   =   0       L   -   1       ⁢           ⁢             d   ^     l     ⁡     (     n   ,   k     )       ·       ξ   )     l       ⁢     (     ⁢   k   ⁢       )     *           ,     0   ≤   n   ≤     M   -   1               [     Equation   ⁢           ⁢   14     ]               
Herein,
     L: a number of paths   
   ξ 1 (k): channel estimation 
   M: a number of total symbols in a slot of the DPCH 
   A second calculating unit  220  and a third calculating unit  230  may respectively calculate a reciprocal of the signal power attenuation ratio of the CPICH to compensate for the attenuation of the signal power of the DPCH signal (equation 12) that may be caused by the fading. 
   The second calculating unit  220  may include a power calculator and a squarer to calculate an average power of the CPICH slot as per equation 7. 
   The third calculating unit  230  may include a squarer and a power calculator to calculate an average power per slot of a square of the CPICH symbol as per equation 9. 
   Results of the second and third calculating units  220  and  230  are respectively applied to filter units  225  and  235 . The filter units  225  and  235  may include a low pass filter (not shown) that may be implemented as a first order filter. The filter units  225  and  235  are used to stabilize the effect of the fading to improve the jitter characteristics of SIR, as described above. 
   The outputs of the filter units  225  and  235  may be provided to a division unit  236  to generate the reciprocal of the power attenuation ratio of the CPICH signal that may be used to compensate for the power attenuation of the DPCH. 
   
     
       
         
           
             
               
                 
                   
                     
                       δ 
                       ^ 
                     
                     l 
                   
                   ⁡ 
                   
                     ( 
                     k 
                     ) 
                   
                 
                 = 
                 
                   
                     
                       
                         U 
                         ~ 
                       
                       
                         cp 
                         , 
                         l 
                       
                     
                     ⁡ 
                     
                       ( 
                       k 
                       ) 
                     
                   
                   
                     
                       
                         S 
                         ~ 
                       
                       
                         cp 
                         , 
                         l 
                       
                     
                     ⁡ 
                     
                       ( 
                       k 
                       ) 
                     
                   
                 
               
             
             
               
                 [ 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   15 
                 
                 ] 
               
             
           
         
       
     
   
   In equation 15, a numerator on the right hand side of equal sign represents the average power per slot of a square of the CPICH symbol (equation 9) passing through the filter unit  235 , which may be expressed by the following equation 16: 
   
     
       
         
           
             
               
                 
                   
                     
                       U 
                       ~ 
                     
                     
                       cp 
                       , 
                       l 
                     
                   
                   ⁡ 
                   
                     ( 
                     k 
                     ) 
                   
                 
                 = 
                 
                   
                     
                       ( 
                       
                         1 
                         - 
                         α 
                       
                       ) 
                     
                     · 
                     
                       
                         
                           U 
                           ~ 
                         
                         
                           cp 
                           , 
                           l 
                         
                       
                       ⁡ 
                       
                         ( 
                         
                           k 
                           - 
                           1 
                         
                         ) 
                       
                     
                   
                   + 
                   
                     α 
                     · 
                     
                        
                       
                         
                           1 
                           
                             S 
                             1 
                           
                         
                         ⁡ 
                         
                           [ 
                           
                             
                               ∑ 
                               
                                 n 
                                 = 
                                 
                                   N 
                                   p 
                                 
                               
                               
                                 
                                   S 
                                   1 
                                 
                                 - 
                                 1 
                               
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               
                                 [ 
                                 
                                   
                                     c 
                                     l 
                                   
                                   ⁡ 
                                   
                                     ( 
                                     
                                       n 
                                       , 
                                       k 
                                     
                                     ) 
                                   
                                 
                                 ] 
                               
                               2 
                             
                           
                           ] 
                         
                       
                        
                     
                   
                 
               
             
             
               
                 [ 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   16 
                 
                 ] 
               
             
           
         
       
     
   
   Herein, Np represents a number of pilot symbols in a slot of the DPCH. In equation 15, a denominator on the right hand side equal sign represents the average power per slot of the CPICH (equation 7) passing through the filter unit  225 , which may be expressed by equation 17: 
   
     
       
         
           
             
               
                 
                   
                     
                       S 
                       ~ 
                     
                     
                       cp 
                       , 
                       l 
                     
                   
                   ⁡ 
                   
                     ( 
                     k 
                     ) 
                   
                 
                 = 
                 
                   
                     
                       ( 
                       
                         1 
                         - 
                         α 
                       
                       ) 
                     
                     · 
                     
                       
                         
                           S 
                           ~ 
                         
                         
                           cp 
                           , 
                           l 
                         
                       
                       ⁡ 
                       
                         ( 
                         
                           k 
                           - 
                           1 
                         
                         ) 
                       
                     
                   
                   + 
                   
                     α 
                     · 
                     
                        
                       
                         
                           1 
                           
                             S 
                             1 
                           
                         
                         ⁡ 
                         
                           [ 
                           
                             
                               ∑ 
                               
                                 n 
                                 = 
                                 
                                   N 
                                   p 
                                 
                               
                               
                                 
                                   S 
                                   1 
                                 
                                 - 
                                 1 
                               
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               
                                 [ 
                                 
                                   
                                     c 
                                     l 
                                   
                                   ⁡ 
                                   
                                     ( 
                                     
                                       n 
                                       , 
                                       k 
                                     
                                     ) 
                                   
                                 
                                 ] 
                               
                               2 
                             
                           
                           ] 
                         
                       
                        
                     
                   
                 
               
             
             
               
                 [ 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   17 
                 
                 ] 
               
             
           
         
       
     
   
   Herein, Np represents a number of pilot symbols in a slot of the DPCH. The average signal power in each slot of DPCH (equation 12) passing through the first calculating unit  210  and the filter unit  215  may be multiplied with the reciprocal of the signal power attenuation ratio of the CPICH (equation 15) output from the division unit  236  by a multiplication unit  217 . 
   The multiplication unit  217  may output an average signal power per slot of the DPCH that compensates for the fading. The average signal power per slot of the DPCH may be calculated by equation 18:
 
Ŝ dp, l (k)={tilde over (S)} dp, l (k)·{circumflex over (δ)} l (k)  [Equation 18]
 
   The DPCH data symbol  207  that compensates for a phase error with respect to each respective path may be then rake combined and a tentative decision is made for the DPCH data symbol  207 . The DPCH data symbol  207  may be provided to a fourth calculating unit  240 , in addition to being provided to the first calculating unit  210 . 
   The fourth calculating unit  240  may include a squarer and an average power calculator to calculate an average power per slot of a square of the DPCH symbol. 
   A result of the fourth calculating unit  240  may be provided to a filter unit  245 . The filter unit  245  may include a low pass filter (not shown) implemented as a first order linear filter of the infinite impulse response (IIR) type for use in stabilizing the effect of the fading to improve the jitter characteristics of SIR, as described above. 
   A subtraction unit  247  subtracts an output of the multiplication unit  217  with an output of the filter unit  245 . Therefore, an output of the subtraction unit  247  may be an interference power of the DPCH. The output of the subtraction unit  247  may be calculated by equation 19:
 
Î l (k)=Ũ dp, l (k)−Ŝ dp, l (k)  [Equation 19]
 
Herein,
 
   
     
       
         
           
             
               
                 U 
                 ~ 
               
               
                 dp 
                 , 
                 l 
               
             
             ⁡ 
             
               ( 
               k 
               ) 
             
           
           = 
           
             
               
                 ( 
                 
                   1 
                   - 
                   α 
                 
                 ) 
               
               · 
               
                 
                   
                     U 
                     ~ 
                   
                   
                     dp 
                     , 
                     l 
                   
                 
                 ⁡ 
                 
                   ( 
                   
                     k 
                     - 
                     1 
                   
                   ) 
                 
               
             
             + 
             
               α 
               · 
               
                  
                 
                   [ 
                   
                     
                       1 
                       M 
                     
                     ⁢ 
                     
                       
                         ∑ 
                         
                           n 
                           = 
                           0 
                         
                         
                           M 
                           - 
                           1 
                         
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           [ 
                           
                             
                               
                                 s 
                                 l 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   n 
                                   , 
                                   k 
                                 
                                 ) 
                               
                             
                             ⁢ 
                             
                               ⅇ 
                               
                                 
                                   - 
                                   j 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   
                                     ϕ 
                                     ^ 
                                   
                                   ⁡ 
                                   
                                     ( 
                                     
                                       n 
                                       , 
                                       k 
                                     
                                     ) 
                                   
                                 
                               
                             
                           
                           ] 
                         
                         2 
                       
                     
                   
                   ] 
                 
                  
               
             
           
         
       
     
       
       M: a number of total symbols in a slot of the DPCH 
     
  
   The output of the subtraction unit  247  may be converted to an absolute value by an absolute value calculating unit  248 , and the absolute value may be provided to a filter unit  250 . The filter unit  250  may include a low pass linear filter (not shown) implemented as an IIR type first order filter. The filter unit  250  may have a filter coefficient β that is a forgetting factor. In one example embodiment of the present invention, the filter coefficient β may be set as about 0.99375. It is noted that the filter coefficient β may be varied to optimize for each communication system. The interference power passing through the filter unit  250  may be calculated by equation 20:
 
Î l (k)=βÎ l (k−1)+(1−β)Î l (k)  [Equation 20]
 
   The signal power and the interference power of each of the respective paths  251  and  252  are combined before SIR estimation. The signal power and the interference power are represented by equation 21: 
   
     
       
         
           
             
               
                 
                   
                     
                       I 
                       _ 
                     
                     ⁡ 
                     
                       ( 
                       k 
                       ) 
                     
                   
                   = 
                   
                     
                       ∑ 
                       
                         l 
                         = 
                         0 
                       
                       
                         L 
                         - 
                         1 
                       
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
                         
                           I 
                           ^ 
                         
                         l 
                       
                       ⁡ 
                       
                         ( 
                         k 
                         ) 
                       
                     
                   
                 
                 ⁢ 
                 
                   
 
                 
                 ⁢ 
                 
                   
                     
                       S 
                       _ 
                     
                     ⁡ 
                     
                       ( 
                       k 
                       ) 
                     
                   
                   = 
                   
                     
                       ∑ 
                       
                         l 
                         = 
                         0 
                       
                       
                         L 
                         - 
                         1 
                       
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
                         
                           S 
                           ^ 
                         
                         l 
                       
                       ⁡ 
                       
                         ( 
                         k 
                         ) 
                       
                     
                   
                 
               
             
             
               
                 [ 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   21 
                 
                 ] 
               
             
           
         
       
     
   
   The signal power and the interference power combined at each of the respective paths  251  and  252  are divided by a division unit  253  to produce the SIR. The calculated SIR may be represented by equation 22:
 
λ(k)=  S   l  (k)/Ī l (k)  [Equation 22]
 
   In a closed-loop power control according to an example embodiment of the present invention, the SIR estimate for an example embodiment of the present invention may be compared with a reference SIR every period of the closed-loop power control, and a power control signal TPC command may be transmitted to a corresponding receiver or transmitter to allow the corresponding receiver or transmitter to control a receiving power thereof. 
   In a W-CDMA communication system, the closed-loop power control may be performed in a period of 1/1500 sec to control power on a slot-by-slot basis, e.g., every slot, so that the power control signal TPC generated by the SIR estimation according to an example embodiment of the present invention may be transmitted to a corresponding receiver or transmitter every 1/1500 sec. For example, in a forward link closed-loop power control, a mobile station may estimate the SIR of a received signal and transmit the power control signal TPC to a base station to control a transmitting power of the base station. 
     FIG. 3  is a block diagram illustrating an apparatus for measuring SIR according to an example embodiment of the present invention. 
   The apparatus for SIR estimation according to an example embodiment of the present invention may include a first channel signal power estimation unit  310 , a second channel signal power estimation unit  320 , a first channel signal power compensation unit  330 , a first channel interference power estimation unit  340  and a signal-to-interference calculation unit  350 . 
   After a DPCH data symbol  207  compensates for the phase error by a channel estimate with respect to each path, may be rake combined and a tentative decision may be made for the DPCH data symbol  207 , the DPCH data symbol  207  may be input to the first channel signal power estimation unit  310 . The first channel signal power estimation unit  310  may be used to estimate an average signal power per slot of the DPCH. 
   The first channel signal power estimation unit  310  may include an averaging unit  311  to calculate an average power per slot of the DPCH data symbol  207 , a squarer  312  to calculate an average power per slot of the DPCH data symbol  207  by squaring the average power per slot of the DPCH data symbol  207  and a filter unit to stabilize the fading for the average power per slot of a squared DPCH data symbol  207 . The filter unit  313  may include a low pass linear filter that is implemented as an IIR type first order filter as described with respect to  FIG. 2 . The filter unit  313  may be used to stabilize the effect of the fading to improve the jitter characteristics of SIR. 
   The second channel signal power estimation unit  320  may be used to calculate a reciprocal of the CPICH signal power attenuation ratio to compensate for the signal power attenuation in the DPCH. 
   The second channel signal power estimation unit  320  may receive a despread CPICH data symbol  201 . 
   The second channel signal power estimation unit  320  may include a first averaging unit  321  to calculate an average power per slot of the CPICH data symbol  201 , a first squarer  322  to calculate an average power of the CPICH slot by squaring the average power per slot of the CPICH data symbol  201 , and a first filter unit  323  to stabilize the fading for the average power per slot of a squared DPICH data symbol  201 . The first filter unit  323  may include a low pass linear filter that is implemented as an IIR type first order filter. The filter unit  323  may be used to stabilize the effect of the fading to improve the jitter characteristics of SIR. 
   In addition, the second channel signal power estimation unit  320  may include a second squarer  324  to square the CPICH data symbol  201 , a second averaging unit  325  to calculate an average power per slot of the CPICH data symbol  201 , a second filter unit  326  to stabilize the fading for the average power per slot of a squared DPICH data symbol  201 . The second filter unit  326  may include a low pass linear filter that is implemented as an IIR type first order filter. The second filter unit  326  may be used to stabilize the effect of the fading to improve the jinter characteristics of SIR. 
   The first and second filter units  323  and  326  may respectively include a low pass linear filter that is implemented as an IIR type first order filter as described with respect to  FIG. 2 . 
   The second channel signal power estimation unit  320  may further include a division unit  327  to divide an output of the second filter unit  326  by an output of the first filter unit  323  to generate a reciprocal  328  of the CPICH signal power attenuation ratio. 
   The first channel signal power compensation unit  330  may be used to compensate for the fading effect on an average signal power  314  of the DPCH slot output from the first channel signal power estimation unit  310 . 
   The first channel signal power compensation unit  330  may include a multiplication unit  331  that multiplies the reciprocal  328  of the CPICH signal power attenuation ratio output from the second channel signal power estimation unit  320  by the average signal power  314  of the DPCH slot output from the first channel signal power estimation unit  310 . Therefore, the first channel signal power compensation unit  330  may output a compensated average power  332  per slot of the first channel. 
   The first channel interference power estimation unit  340  may be used to estimate the interference power of the DPCH for SIR estimation. 
   The first channel interference power estimation unit  340  may receive the DPCH data symbol  207  that compensates for a phase error with respect to each path, may be rake combined and for which a tentative decision may be made. 
   The first channel interference power estimation unit  340  may include a squarer  341  to square the DPCH data symbol  207 , an averaging unit  342  to calculate an average power per slot of the DPCH data symbol  207 , a first filter unit  343  to stabilize the fading for the average power per slot of the squared DPCH data symbol  207 . The first filter unit  343  may include a low pass filter that is implemented as an IIR type first order filter as described with respect to  FIG. 2 . 
   The first channel interference power estimation unit  340  may further include a subtraction unit  344 , an absolute value calculating unit  345 , and a second filter unit  346 . 
   The subtraction unit  344  may subtract the compensated average power  332  from an output of the first filter unit  343  to generate the interference power of the DPCH channel. 
   The absolute value calculating unit  345  may generate an absolute value of an output of the subtraction unit  344 . The filter unit  346  that receives the output of the subtraction unit  344  may include a low pass filter implemented as an IIR type first order filter having a forgetting factor. 
   The signal-to-interference ratio (SIR) calculation unit  350  may combine the DPCH channel signal power and the interference power for respective associated paths to generate the SIR. 
   The signal-to-interference ratio (SIR) calculation unit  350  may include a first summer  351 , a second summer  352 , and a division unit  353 . 
   The first summer  351  may sum respective compensated average power  332  per slot for a DPCH channel for respective associated paths. The second summer  352  may sum respective interference power of the DPCH channel for respective associated paths. The division unit  353  may divide an output of the first summer  351  by an output of the second summer  352  to output the SIR  360 . 
   It is very well known in the art that a system module may be easily combined with other various components and functions. Therefore, it will be understood that portions of elements according to example embodiments of the present invention may be combined to form a functional group, which may be referred to by a different name. 
   Therefore, it is noted that the respective elements of the apparatus for SIR estimation shown in  FIG. 3  are given only for illustrative purposes as each functional group and therefore, two or more elements may be combined to form a new functional group referred to by a different name. 
     FIG. 4  is a graph illustrating a signal power of a DPCH signal according to an example embodiment of the present invention. 
   Referring to  FIG. 4 , the signal power attenuation of the DPCH may be compensated for by the SIR estimation according to an example embodiment of the present invention. For a mobile device moving at a speed of about 120 km/h ( 202 ) and a mobile device moving at a speed of about 250 km/h ( 203 ), the signal power attenuation due to fading may be compensated to exhibit an accurate signal power attenuation. Namely, compared with an ideal case where there no fading exists ( 201 ), there is little difference for  202  and  203 . 
   Therefore, not only in a slow fading but also in a fast fading environment, the SIR may be accurately estimated by compensating for the signal power attenuation of the DPCH. 
     FIGS. 5A and 5B  are graphs showing measured SIR results for a conventional SIR estimation method and for an example embodiment of the present invention. 
   In  FIG. 5A , the SIR measurement according to the conventional SIR estimation may be carried out without fading ( 501   a ), for a mobile device moving at a speed of about 3 km/h ( 502   a ), for a mobile device moving at a speed of about 50 km/h ( 503   a ), for a mobile device moving at a speed of about 120 km/h ( 504   a ) and for a mobile device moving at a speed of about 250 km/h ( 505   a ). 
   Similarly, referring to  FIG. 5B , the SIR measurement according to an example embodiment of the present invention may be carried out without fading ( 501   b ), for a mobile device moving at a speed of about 3 km/h ( 502   b ), for a mobile device moving at a speed of about 50 km/h ( 503   b ), for a mobile device moving at a speed of about 120 km/h ( 504   b ) and for a mobile device moving at a speed of about 250 km/h ( 505   b ). 
   Comparing the SIR characteristics as illustrated in  FIGS. 5A and 5B , according to an example embodiment of the present invention an error in the SIR estimation is reduced more as compared to that of the prior art. 
   Particularly, with respect to a high-speed moving mobile device, the error in the SIR estimation for example embodiments of the present invention is less than 2 dB, whereas the maximum error in the SIR estimation is greater than about 10 dB as the Es/No increases. Therefore, according to an example embodiment of the present invention, the maximum error in the SIR estimation is reduced by 8 dB. 
     FIGS. 6A and 6B  are graphs illustrating SIR jitter characteristics for a conventional SIR estimation method and for an example embodiment of the present invention. 
   Referring to  FIG. 6A , the SIR jitter characteristics for the conventional SIR estimation are measured with respect to a mobile device moving at a speed of about 3 km/h ( 601   a ), a mobile device moving at a speed of about 50 km/h ( 602   a ), a mobile device moving at a speed of about 120 km/h ( 603   a ), and a mobile device moving at a speed of about 250 km/h ( 604   a ). 
   Similarly, referring to  FIG. 6B , the SIR average characteristics are shown with respect to a mobile device moving at a speed of about 3 km/h ( 601   b ), a mobile device moving at a speed of about 50 km/h ( 602   b ), a mobile device moving at a speed of about 120 km/h ( 603   b ), and a mobile device moving at a speed of about 250 km/h ( 604   b ). 
   Comparing the SIR jitter characteristics in  FIGS. 6A and 6B  reveals that the SIR estimates according to an example embodiment of the present invention has overall improved jitter characteristics by about 1 dB than that of the prior art. 
     FIG. 7  is a table illustrating conditions of various parameters in simulations for  FIGS. 4 ,  5 B and  6 B. 
   The parameters illustrates in  FIG. 7  conform to the 3GPP standard and are used in the simulation for the SIR estimation in  FIGS. 4 ,  5 B and  6 B. 
   According to an example embodiment of the present invention, in a closed-loop power control of the direct sequence code division multiple access (DS-CDMA) system, the power attenuation of a signal on a channel that is to be estimated may be compensated for so that the accuracy of the SIR estimation may be enhanced under the fading environment. Therefore, the power control of the closed-loop may be improved. 
   In addition, the SIR estimate may be used as status information for selecting a modulation and coding scheme (MCS) in a high speed downlink packet access (HSDPA) to improve the performance of the HSDPA.