Abstract:
A method of calculating the noise power in a received signal containing a pilot channel, the method comprising estimating the power of the received signal, estimating the power of the pilot channel and calculating the noise power as a function of the estimated signal and the pilot channel powers. The noise power thus deduced may be used as a parameter of a function defining a channel quality indicator. The invention extends to corresponding apparatus.

Description:
BACKGROUND 
     The present invention relates the assessment of noise in received communications signals and to the assessment of channel quality. 
     In 3GPP TS 25.214 V6.7.1 “Physical layer procedures (FDD)”, the UE is required to report the channel quality indicator (CQI) for HS-DSCH rate adaptation. The definition of CQI is given as follows (TS 25.214 Section 6A.2): 
     “Based on an unrestricted observation interval, the UE shall report the highest tabulated CQI value for which a single HS-DSCH sub frame formatted with the transport block size, number of HS-PDSCH codes and modulation corresponding to the reported or lower CQI value could be received in a 3-slot reference period ending 1 slot before the start of the first slot in which the reported CQI value is transmitted and for which the transport block error probability would not exceed 0.1.” 
     The CQI values are tabulated according to UE category and have been designed to correspond to 1 dB steps in the required Signal-to-Interference plus Noise Ratio (SINR) that is required to achieve the specified BLER of 10%. In static conditions such as those implied by the definition of CQI, the relationship between the SINR and the corresponding CQI value has been established as being linear (see, for example, Brouwer, et al., “Usage of link-level performance indicators for HSDPA network-level simulations in E-UMTS,”  Spread Spectrum Techniques and Applications,  2004  IEEE Eighth International Symposium on , pp. 844-848). 
     The UE is also supplied with the power offset between the HS-DSCH channels and the CPICH (Common Pilot Channel) and thus the determination of the CQI value can be based on the CPICH SINR. 
     Methods for estimating the CPICH SINR based on the despread CPICH symbols are known in the art, see e.g. “CPICH Processing for SINR Estimation in W-CDMA System”, WO2005093961. 
     SUMMARY OF THE INVENTION 
     According to one aspect, the invention provides a method of calculating the noise power in a received signal containing a pilot channel, the method comprising estimating the power of the received signal, estimating the power of the pilot channel and calculating the noise power as a function of the estimated signal and pilot channel powers. 
     Noise measurements made in this way can be used to evaluate a CQI without processing the CPICH. Avoiding a need to process the CPICH reduces delay in CQI estimation since processing the CPICH will delay the CQI estimation by at least the CPICH spreading factor. The computational load can also be reduced in the need to process the CPICH can be avoided. 
     In certain embodiments, the function defining the noise power has a form that can be attained by applying a linear regression technique to an expression linking the noise power, the received signal power and the pilot channel power. 
     In certain embodiments, the pilot channel power is obtained from magnitudes of tap values in an estimate of the response of the channel through which the signal is acquired. 
     In certain embodiments, the signal is transmitted using a transmit diversity scheme and the estimated pilot channel power is obtained by summing pilot channel powers estimated for elements of the signal issuing from different locations. 
     In certain embodiments, the function defining the noise power is a function of power estimates of the pilot channel arising at different time-points. 
     In certain embodiments the function defining the noise power is a function of power estimates of the received signal arising at different time-points. 
     The noise power as determined using the techniques of the invention can be used to calculate a channel quality indicator for a received signal by evaluating a function for said indicator having the noise power as an argument. The noise power may be filtered or averaged prior to being used to evaluate the function defining the CQI. The function defining the CQI may also have as an argument the estimated power of a pilot channel contained within the signal. The pilot channel power can be filtered or averaged prior to being used in evaluating the function defining the CQI. The function defining the CQI may also have as an argument the estimated power of the received signal. The power of the received signal can be filtered or averaged prior to being used to evaluate the function defining the CQI. The form of the function defining the CQI can vary in dependence upon the power of the received signal. 
     Above, certain elements of the invention have been described from the method perspective. For the avoidance of doubt, the invention also extends to apparatus for and also to programs for performing methods according to the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       By way of example only, certain embodiments of the invention will now be described, by reference to the accompanying drawings, in which: 
         FIG. 1  illustrates schematically a UE device; and 
         FIG. 2  illustrates schematically processing stages in the CQI calculation unit of the UE device of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts various processing stages that form part of a UE such as a mobile telephone. It should be noted that the blocks shown in  FIG. 1  represent processing operations performed on a received signal but do not necessarily correspond directly to physical units that may appear within a practical implementation of a receiver (the same applies to the processing stages shown in  FIG. 2 ). The first stage  101  corresponds to the radio frequency processing. During the radio frequency processing, the received signal is down-converted to base-band using a mixer  103 . The reference frequency used to drive the mixer is generated by an oscillator  104 . Following this carrier down-conversion, the signal is low-pass filtered  102  and then passed to the mixed-signal processing stage  108 . 
     The mixed signal processing includes Analogue-to-Digital Conversion (ADC)  105 , sampling  106  and low pass filtering  107 . The resulting signal, which is now digital, is supplied to the digital signal processing stage  111  where it is processed such that the transmitted information can be recovered. The received signal is then processed by the CQI estimation unit  109 , the different processing stages of which are presented in  FIG. 2 . 
     The received signal is a W-CDMA chip stream that has passed through a transmission link and can be expressed as: 
               r   ⁡     (   k   )       =         α   cpich     ⁢       ∑     l   =   0     P     ⁢       h   ⁡     (   l   )       ⁢     c   ⁡     (     k   -   l     )             +       ∑     l   =   0     P     ⁢       h   ⁡     (   l   )       ⁢       ∑     m   =   1     M     ⁢       α   m     ⁢       s   m     ⁡     (     k   -   l     )               +     ζ   ⁡     (   k   )               
where {h(k)} kε{0, . . . , P}  represent the different propagation channel taps. ζ(k) models the combination of thermal noise and interference from adjacent cells. ζ(k) is assumed to be Additive White Gaussian Noise (AWGN) with variance equal to σ 2 . The samples c(k) denote the unit amplitude CPICH sequence transmitted with amplitude α cpich  and s m  (k) is the unit amplitude data sequence for the mth channel transmitted with amplitude α m .
 
     In CQI estimation unit  109 , the received signal is first processed by unit  200  in order to generate raw channel estimates. These initial channel estimates can be generated, for example, by correlating the received signal with the known pilot sequence as described in the above equations. It should however be noted that the application of the present invention is not restricted to this case. It would be possible to use other techniques, such as linear Least-Square fitting ( Digital Communications , John G. Proakis, 2 nd    edition , McGraw-Hill International), in order to derive these channel estimates. 
     In case of transmit diversity, either open-loop or closed-loop, the unit  200  will generate a set of initial channel estimates for each transmit antenna. 
     The CIR is derived by correlating the received signal with the pilot sequence: 
                       h   ^     ⁡     (   l   )       =       1   N     ⁢       ∑     k   =   0       N   -   1       ⁢       r   ⁡     (   k   )       ⁢       c   *     ⁡     (     k   -   l     )                         =       α   cpich     ⁢     h   ⁡     (   l   )                     
where N is the spreading factor of the W-CDMA pilot signal. It is assumed, without loss of generality, that
 
                 ∑     l   =   0     P     ⁢            h   ⁡     (   l   )            2       =   β         
where β represents the fading loss (or gain) of the transmission channel.
 
     The received signal is also processed by unit  201  to calculate the received signal power level in the following manner: 
     
       
         
           
             
               P 
               rx 
             
             = 
             
               
                 1 
                 T 
               
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     The generated channel estimates are processed by unit  202  in order to generate an estimate of the received CPICH power level. 
     Since the generated channel estimates are scaled by the magnitude of the CPICH channel, an estimate of the received CPICH power level can be calculated in the following manner: 
     
       
         
           
             
               P 
               cpich 
             
             = 
             
               
                 ∑ 
                 
                   l 
                   = 
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                 P 
               
               ⁢ 
               
                 
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     P cipch  can also be formulated as:
 
P cpich α cpich   2 β
 
     This can be rearranged to give: 
     
       
         
           
             β 
             = 
             
               
                 P 
                 cpich 
               
               
                 α 
                 cpich 
                 2 
               
             
           
         
       
     
     In the case of transmit diversity, either open-loop or closed-loop, the total received CPICH power level is calculated as the sum of the CPICH power estimated for each transmit antenna. 
     In one embodiment of the invention, unit  203  processes the received signal power and the estimated CPICH power to calculate the noise power. The noise power can be calculated based on the following observation; if the received power, P rx , is calculated over the same interval as used to calculate the channel estimates, the following relationship holds:
 
 P   rx ( i )= P   t β( i )+σ 2  
 
     And we can substitute for β to give: 
                 P   rx     ⁡     (   i   )       =           P   t       α   cpich   2       ⁢       P   cpich     ⁡     (   i   )         +     σ   2             
where P t  is the total transmitted power level:
 
               P   t     =       α   cpich   2     +       ∑     m   =   1     M     ⁢     α   m   2               
which is assumed to remain constant and σ 2  is the noise power. The index, i, represents the ith observation of the two quantities.
 
     One method of calculating the noise power based on two or more observations of P rx (i) and P cpich (i) is by solving the linear equation:
 
 P   rx   =αP   cpich +σ 2  
 
where
 
 P   rx   =[P   rx ( i ), P   rx ( i− 1), . . . ,  P   rx ( i−k +1)] T  
 
 P   cpich   =[P   cpich ( i ), P   cpich ( i− 1), . . . ,  P   cpich ( i−k+ 1)] T  
 
with k being the number of observations. Reformulating the above equation gives:
 
               P   rx     =         [       P   cpich     ⁢           ⁢   1     ]     ⁡     [         α             σ   2           ]       =   Xv           
which is solved for the two unknowns as:
 
 v =( X   T   X ) −1   X   T   P   rx  
 
     For the simplest case of 2 observations (k=2), this gives the result: 
     
       
         
           
             
               
                 
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     During a CQI measurement period, the three measures, P rx , P cpich  and {circumflex over (σ)} 2 , will be calculated multiple times and in one embodiment of the invention, the average quantities: 
                 P   _     rx     =       1   K     ⁢       ∑     i   =   1     K     ⁢       P   rx     ⁡     (   i   )                           P   _     cpich     =       1   K     ⁢       ∑     i   =   1     K     ⁢       P   cpich     ⁡     (   i   )                         N   o     =       1   K     ⁢       ∑     i   =   1     K     ⁢         σ   ^     2     ⁡     (   i   )                 
are calculated by unit  204 .
 
     Based on these three measures, unit  204  calculates a CQI value for the nth measurement period as:
 
 CQI ( n )= f (   P     rx ( n ),   P     cpich ( n ), N   o ( n ))
 
where the mapping, f(●), can be determined by experimentation. In one embodiment of the invention, mapping f(●) is modelled as a linear relationship:
 
               CQI   ⁡     (   n   )       =         Λ   ·   10     ⁢           ⁢       log   10     ⁡     (           P   _     cpich     ⁡     (   n   )           N   o     ⁡     (   n   )         )         +   Υ   +   Γ           
where Λ and Υ are determined by experimentation and Γ is the difference between the CPICH power level and the HS-DSCH power level. It will be recalled from the introduction that Γ is a known parameter that is signalled to the UE.
 
     The linear relationship described above may not accurately describe the mapping f(●) and thus another embodiment of the invention uses the following transfer function: 
               CQI   ⁡     (   n   )       =     {                 Λ   1     ·   10     ⁢           ⁢       log   10     ⁡     (           P   _     cpich     ⁡     (   n   )           N   o     ⁡     (   n   )         )         +     Υ   1     +   Γ                 P   _     rx     ⁡     (   n   )       &lt;   Δ                     Λ   2     ·   10     ⁢           ⁢       log   10     ⁡     (           P   _     cpich     ⁡     (   n   )           N   o     ⁡     (   n   )         )         +     Υ   2     +   Γ                 P   _     rx     ⁡     (   n   )       ≥   Δ                   
where Δ is determined by experimentation and represents the received signal level at which the CQI mapping changes.
 
     This approach can clearly be extended to any approximation of the actual CQI mapping, including, but not limited to using one or more lookup tables.