Abstract:
To evaluate an energy level of a signal received through a radio interface between radiocommunication stations, successive measurement samples depending on the energy of the signal are obtained, and an average of said samples is calculated, to evaluate the energy level. The calculation of the average of the samples is performed with an averaging duration depending on a service to which the received signal pertains.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the energy measurements performed in mobile radiocommunication systems. 
     Such measurements are usable in particular in radio link control procedures which serve in a general manner to optimize the quality of the transmissions and to minimize the interference between users. Among these procedures mention may be made of the regulating of transmission power by the mobiles and/or by the infrastructure, the handover control in cellular networks, the adaptation of the channel coding to the propagation conditions, the adaptation of the source coding (in particular the case of variable rate speech encoders of AMR “Adaptive Multi-Rate” type), the link adaptation procedures, etc. 
     The efficiency of this kind of procedure depends on the availability of reliable energy measurements within as short a timespan as possible, so that suitable decisions can be taken fairly quickly. 
     Moreover, allowance for point-like measurement samples may disturb the control algorithms implemented, by reason of the short-term fluctuations of the propagation channel. This is why time smoothing of these samples is generally performed by means of an averaging window of duration equal to N times the period between samples. The window may be rectangular: an arithmetic average of the last N samples is then computed. It is common to employ an exponential window with a forgetting factor α=1/N: upon receipt of an n-th sample C(n), a smoothed value  C (n)=(1−α).  C (n−1)+α·C(n) is then produced. 
     A long duration of the averaging window decreases the standard deviation of the estimate of the measured parameter, and therefore improves the reliability of the measurement. Conversely, a short duration increases the speed of acquisition of the measurements and therefore makes it possible, to the extent that they are reliable, to optimize the performance of the control algorithm which utilizes them. The choice of this duration results from a compromise between these two contradictory requirements. 
     An object of the present invention is to propose a finer way of making this compromise. 
     SUMMARY OF THE INVENTION 
     The invention thus proposes a method of evaluating an energy level of a signal received through a radio interface between radiocommunication stations, wherein successive measurement samples depending on the energy of the signal are obtained, and an average of said samples is calculated to evaluate the energy level. According to the invention, the calculation of the average of said samples is performed with an averaging duration depending on a service to which the received signal pertains. 
     In particular, the averaging duration may increase as a function of the information throughput carried by the signal received. 
     In a preferred embodiment of the invention, a speed of movement of one the stations is estimated, and the averaging duration likewise depends on the estimated speed. 
     The standard deviation of the energy level estimator depends on the duration of averaging and on the speed of the mobile station. It is generally a decreasing function of the duration of averaging and of the speed. By tailoring the duration of averaging as a function of speed, it is possible to target a particular value of standard deviation and hence to meet the requirements of a given application, such as a radio link control procedure. 
     The speed of movement of the mobile station may be estimated by various means. In an advantageous embodiment, it is estimated on the basis of the time variations of the measurement samples. 
     Another aspect of the invention relates to a device for evaluating an energy level of a signal received by a mobile radiocommunication station, comprising measurement means for obtaining successive samples depending on the energy of the signal, and means for calculating an average of said samples to evaluate the energy level, the average calculation means being controlled to cause a duration of averaging of the samples to depend on a service to which the received signal pertains. 
     A third aspect of the invention concerns a device for evaluating an energy level of a signal received by a fixed radiocommunication station from a mobile station, comprising measurement means for obtaining successive samples depending on the energy of the signal, and means for calculating an average of said samples to evaluate the energy level, the average calculation means being controlled to cause a duration of averaging of the samples to depend on a service to which the received signal pertains. 
     A fourth aspect of the invention concerns a method of evaluating an energy level of a signal received from a mobile station by a fixed station through a radio interface between said stations, wherein successive measurement samples depending on the energy of the signal are obtained, an average of said samples is calculated to evaluate the energy level, and a speed of movement of the mobile station is estimated, the calculation of the average of said samples being performed with an averaging duration depending on the estimated speed. According to this aspect of the invention, the average calculation comprises a first averaging performed in the fixed station with a first averaging duration less than said duration dependent on the estimated speed, and a second averaging performed in an item of equipment controlling the fixed station with said averaging duration dependent on the estimated speed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of an exemplary radiocommunication receiver implementing the method according to the invention; 
         FIG. 2  is a graph illustrating charts usable in the method according to the invention; 
         FIG. 3  is diagram illustrating elements of a UMTS type radiocommunication system; and 
         FIGS. 4 to 6  are diagrams illustrating various possible distributions of means used by the method between the elements of  FIG. 3 . 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention is described hereinbelow in its non-limiting application to code division multiple access (CDMA) cellular networks of UMTS (Universal Mobile Telecommunications System) type. 
     In a CDMA system, the symbols sent, binary (±1) or quaternary (±1 ±j), are multiplied by spreading codes composed of samples, called “chips”, whose rate is greater than that of the symbols. Orthogonal or quasi-orthogonal spreading codes are allotted to the various logical channels sharing the same carrier frequency, so as to allow each receiver to detect the sequence of symbols which is destined therefor, by multiplying the signal received by the corresponding spreading code. 
     The rake receiver performs coherent demodulation based on an approximation of the impulse response of the radio propagation channel by a series of peaks, each peak appearing with a delay corresponding to the propagation time along a particular path and having a complex amplitude corresponding to the attenuation and to the phase shift of the signal along this path (instantaneous realization of fading). By analyzing several reception paths, i.e. by sampling, several times, the output from a filter adapted to the spreading code of the channel, with delays corresponding respectively to these paths, the rake receiver obtains multiple estimates of the symbols sent, which are combined to obtain diversity gain. Combining can be performed in particular according to the so-called MRC (Maximum Ratio Combining) method, which weights the various estimates as a function of the complex amplitude observed for the various paths. 
     In order to allow this coherent demodulation, pilot symbols are provided for the estimation of the impulse response in the form of a succession of peaks. The impulse response is estimated by means of a filter matched to a particular spreading code, which may be the spreading code of the communication in progress or that allotted specially to a pilot channel, with which the transmitter modulates a sequence of known symbols, for example symbols set to 1. The positions of the maxima of the output from this matched filter give the delays used in the fingers of the rake receiver, and the associated complex amplitudes correspond to the values of these maxima. 
     The CDMA receiver represented in  FIG. 1  comprises a radio stage  1  which receives the radio signal picked up by the antenna  2  and transposes it to lower frequency. The resulting signal is digitized by an analogue/digital converter  3 , then supplied to a reception filter  4 . The filter  4  carries out filtering matched to the shaping of the signals by the transmitter. It delivers a digital signal Y at the rate of one complex sample per chip of the spreading codes. 
     In the case where the pilot symbols are time-multiplexed with the information symbols (the downlink case in the UMTS system), a demultiplexer  5  distributes the signal Y between a channel probing module  6 , which operates on the signal corresponding to the pilot symbols, and a rake receiver  7  which processes the remainder of the signal with the aid of parameters characteristic of the channel which are provided by the probing module  6 . The pilot symbols may also be located on a different channel than the information symbols (the uplink case in the UMTS system). 
     The probing module  6  estimates in a manner known per se propagation delays τ 1 , . . . τ n  corresponding to multiple propagation paths between the transmitter and the receiver. To do this, it performs a matched filtering of the signal Y by the product of the pilot symbols and spreading code c(t), and it detects the time positions of the n strongest peaks at the output from this matched filter. The complex amplitudes of these spikes A 1 , . . . , A n  are provided to the rake receiver  7  along with the delays τ 1 , . . . , τ n . 
     The receiver  7  also comprises a matched filter  8  which multiplies the signal received Y by the channel spreading code c(t) or its conjugate. Each of the delays τ i  (1≦i≦n) is applied to the output of this matched filter  8 , and the signal thus delayed is multiplied by the complex conjugate A i * of the complex amplitude A i  provided by the probing module  6 . In accordance with the MRC process, the n contributions resulting therefrom are summed by an adder  11  to obtain the estimated information symbols {circumflex over (b)}. 
     Among the parameters available in the probing module  6 , there is the energy C k  of a signal block received on the n propagation paths considered. This energy C k , referred to the duration of a bit, is given by the sum of the squared moduli of the amplitudes A i : 
                     C   k     =       1   N     ⁢       ∑     i   =   1     n     ⁢       β   k     ·            A   i          2                   (   1   )               
where N is the number of pilot symbols of the relevant signal block and β k  is a weighting coefficient which takes account of the amplification or of the attenuation introduced, as the case may be, by the transmission power control procedures.
 
     The receiver is thus furnished with an energy measurement sample C per received signal block. In the dedicated channels of the UMTS system, this block corresponds to a time interval of 2560 chips, i.e. 666 μs, and the number N may range from 2 to 16 (see sections 5.2.1 and 5.3.2 of the technical specification 3G TS 25.211 v3.3.0 “Physical channels and mapping of transport channels onto physical channels (FDD) (Release 1999)” published in June 2000 by the 3GPP (“Third Generation Partnership Project”)). 
     On the basis of these measurement samples C k , it is possible to estimate the speed v of movement of the mobile station. This estimation can take place in the mobile station on the basis of the downlink signal, or in the fixed station on the basis of the uplink signal. 
     The speed estimation can in particular utilize the property that the variance of the first derivative of C k  with respect to time is the additive inverse of the second derivative of the autocorrelation function R CC (τ) of the energy C k  for a time offset τ=0, this second derivative itself being proportional to the square of the speed. Within the context of the Jake model and for a signal with energy a, the proportionality constant equals 
                 -     (       2   ⁢           ⁢     π   2     ⁢     v   2         λ   2       )       ⁢     a   2       ,         
λ being the radio wavelength used.
 
     In practice, the module  12  can estimate the speed of movement v in the following manner. One is given a duration over which the speed estimation applies, expressed as a number of measurement samples p. The energy a is estimated via the average of the samples C k , indexed by an integer k in equation (2): 
     
       
         
           
             
               
                 
                   a 
                   = 
                   
                     
                       1 
                       p 
                     
                     ⁢ 
                     
                       
                         ∑ 
                         
                           k 
                           = 
                           1 
                         
                         p 
                       
                       ⁢ 
                       
                         C 
                         k 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     The module  12  determines the derived sequence of C k : D 1 , D 2 , . . . D p  with D k =(C k −C k−1 )/T e , where T e  is the duration between two measurement samples. The variance of this sequence is estimated by: 
     
       
         
           
             
               
                 
                   S 
                   = 
                   
                     
                       1 
                       p 
                     
                     ⁢ 
                     
                       
                         ∑ 
                         
                           k 
                           = 
                           1 
                         
                         p 
                       
                       ⁢ 
                       
                         D 
                         k 
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     The module  12  can then estimate the speed as: 
     
       
         
           
             
               
                 
                   v 
                   = 
                   
                     
                       1 
                       
                         2 
                       
                     
                     · 
                     
                       λ 
                       
                         π 
                         · 
                         a 
                       
                     
                     · 
                     
                       S 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     The duration p·T e  is typically of the order of magnitude of a second. In practice, the arithmetic averages of relations (2) and (3) may be replaced with an exponential type smoothing with a forgetting factor αα=1/p. 
     The standard deviation of a random process with autocorrelation function R XX , observed over a duration T is given by: 
     
       
         
           
             
               
                 
                   
                     σ 
                     2 
                   
                   = 
                   
                     
                       1 
                       T 
                     
                     ⁢ 
                     
                       
                         ∫ 
                         
                           - 
                           T 
                         
                         T 
                       
                       ⁢ 
                       
                         
                           ( 
                           
                             1 
                             - 
                             
                               τ 
                               T 
                             
                           
                           ) 
                         
                         ⁢ 
                         
                           
                             R 
                             xx 
                           
                           ⁡ 
                           
                             ( 
                             τ 
                             ) 
                           
                         
                         ⁢ 
                         
                           ⅆ 
                           τ 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     In the case of Rayleigh fading and for the Jake model, the autocorrelation function of the energy expressed in dB may be written: 
     
       
         
           
             
               
                 
                   
                     
                       R 
                       xx 
                     
                     ⁡ 
                     
                       ( 
                       τ 
                       ) 
                     
                   
                   = 
                   
                     5 
                     , 
                     
                       57 
                       2 
                     
                     × 
                     
                       
                         J 
                         0 
                         2 
                       
                       ⁡ 
                       
                         ( 
                         
                           
                             2 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             π 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             v 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             τ 
                           
                           λ 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
         
         
           
             where J 0  denotes the Bessel function of order 0. Consequently, the standard deviation σ in dB is given by: 
           
         
       
    
     
       
         
           
             
               
                 
                   σ 
                   = 
                   
                     
                       
                         2 
                         T 
                       
                       ⁢ 
                       
                         
                           ∫ 
                           0 
                           T 
                         
                         ⁢ 
                         
                           
                             ( 
                             
                               1 
                               - 
                               
                                 τ 
                                 T 
                               
                             
                             ) 
                           
                           × 
                           5 
                           , 
                           
                             57 
                             2 
                           
                           × 
                           
                             
                               J 
                               0 
                               2 
                             
                             ⁡ 
                             
                               ( 
                               
                                 
                                   2 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   π 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   v 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   τ 
                                 
                                 λ 
                               
                               ) 
                             
                           
                           ⁢ 
                           
                             ⅆ 
                             τ 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     From this equation (7) we can determine as a function of the speed v the period T of averaging of the energy required in order to achieve a given standard deviation σ. 
     Represented in  FIG. 2  are three curves I, II, III which correspond to the expression (7) for the standard deviation σ as a function of the duration of averaging T for speeds of movement v respectively equal to 2 km/h, 5 km/h and 10 km/h, the wavelength λ corresponding to a carrier frequency of 1950 MHz. 
     If one is given an objective in terms of standard deviation σ, within the context of a procedure using the averaged measurements of C k , it is then possible, starting from the speed v estimated by the module  12 , to determine the duration of averaging T to be taken into consideration. 
     For example, in the case of  FIG. 2 , a standard deviation of 1.5 dB in the energy measurement is obtained with an averaging duration which is shorter the larger the speed: T≈2.2 s for v=2 km/h, T≈0.9 s for v=5 km/h and T≈0.4 for v=10 km/h. 
     This selection of the averaging duration T is performed by the module  13  represented in  FIG. 1 . 
     The averaging duration selected by the module  13  can depend on the service to which the signal transmitted pertains. For certain services, it may in fact be justified to target a smaller standard deviation σ in the energy measurements than for other services. 
     For example, in the multimedia context envisaged in UMTS, the services of relatively high throughput travel through channels with low spreading factor. Consequently, to obtain a given transmission quality in terms of binary error rate, these high throughput channels generally have a larger transmission power than the channels with relatively low throughput. Under these conditions, the relative accuracy of the measurements utilized in the power control procedure is more sensitive for these high throughput services. A lower value of standard deviation σ will therefore be targeted for the high throughput channels, which entails increasing the averaging duration T, for a given speed. 
     The selecting of the averaging duration T can be performed by storing tables corresponding to the curves of  FIG. 2 , and by accessing these tables with the aid of the standard deviation objective a and of the speed v estimated by the module  12 . Of course, it is possible to store only a restricted number of values in the tables, completing the missing values by interpolation when necessary. These tables may be made different from one service to another, in particular as a function of the information throughput carried by the signal. The dependence of the duration T as a function of the service is then achieved by selecting the appropriate table on the basis of the current service. 
     The smoothing of the measurement samples C k  can then be performed by the module  14  represented in  FIG. 1 , preferably with the aid of an exponential window using a forget factor α=T e /T. The smoothed values  C  resulting therefrom may then be used in various radio link control procedures. If the latter require a lower standard deviation σ or are content with a larger standard deviation σ, the duration T is adjusted accordingly by the module  13 . 
     In a known manner, the channel probing module  6  can also deliver measurement samples NO k  of the noise observed on the channel. These samples may also be smoothed by an exponential window filter  15 . To do this, the same forgetting factor α=T e /T as in the filter  14  is advantageously used. 
     If the application has a requirement for estimations of the signal/noise ratio, the smoothing may be applied to the quantity C k /NO k . The energy level taken into account is then normalized with respect to the noise level. 
       FIG. 3  schematically shows the elements of the UTRAN radio access network (UMTS Terrestrial Radio Access Network) forming part of a UMTS network. UTRAN comprises radio network controllers  22  called RNCs linked through a standardized interface lub to fixed stations  21  called “node Bs”. These fixed stations  21  communicate with the mobile stations  20 , called UEs (User Equipment), via CDMA radio links over a standardized interface Uu. The RNCs  22  are linked to a core network (not shown) providing the subscriber management and fixed switching functions. 
     The physical layer protocols relating to the radio links (layer 1 of the ISO model) are installed mainly at the level of the UE  20  and of the node B(s)  21  with which it is in contact. The radio resource control RRC protocol belongs to a higher layer and is located at the level of the UE  20  and of its serving RNC  22 . The RRC instance of the UE  20  communicates with that of its serving RNC  22  by means of an “RRC connection” as described in the technical specification 3G TS 25.331, v3.3.0 “RRC Protocol Specification (Release 1999)” published in June 2000 by the 3GPP. 
     In general, the radio link control procedures are supervised by the RNC  22  according to the RRC protocol. The module  25  represented in  FIGS. 4 to 6  illustrates this RRC layer processing which, within the framework described previously, utilizes the smoothed energy measurements  C . 
     These  FIGS. 4 to 6  show that the modules  12 ,  13 ,  14  described with reference to  FIG. 1  may be distributed in various ways between the communicating entities. 
     The configuration of  FIG. 4  corresponds to the case where these modules  12 ,  13 ,  14  are all located in the station receiving the signal whose power is estimated. This station may be the UE  20 , in which case the measurements relate to the downlink signals and the smoothed measurements  C  go back up to the RNC  22  by way of the RRC connection. The station equipped with the modules  12 ,  13 ,  14  may also be the node B  21 , in which case the measurements relate to the uplink signals and the smoothed measurements  C  go back up to the RNC  22  via the lub interface. 
     In the configuration of  FIG. 5 , the speed estimate made by the module  12  in the UE  20  or the node B  21  is transmitted to the module  13 , situated in the RNC  22 , via the RRC connection or the lub interface. The RNC then determines the averaging duration T and sends it back to the UE  20  or the node B  21  which comprises the smoothing module  14 . This may be performed by adjusting the forgetting factor α, as described in section 8.3.8 of the technical specification 3G TS 25.433, v3.2.0 “UTRAN lub interface NBAP Signalling (Release 1999)” published in June 2000 by 3GPP, and as described in section 8.5.7.7.2 of the aforesaid 3G TS 25.331 specification in the case of the UE  20 . The smoothed measurement  C  obtained by the module  14  is thereafter transmitted to the processing module  25  of the RNC  22 . 
     In the configuration of  FIG. 6 , the smoothing module  14 A situated in the UE  20  or the B node  21  performs a first smoothing of the energy measurement samples C k  with a window of relatively short duration. This duration may be fixed or adjusted according to various criteria by the RNC  22  by means of the mechanism described previously. The smoothed value  C   1  resulting therefrom is conveyed back up to the RNC  22  by the smoothing module  14 A of the UE  20  or of the node B  21 . The RNC  22  comprises a second smoothing module  14 B which applies the averaging duration T selected by the module  13  on the basis of the speed estimate v. This second smoothing module  14 B provides the processing module  25  with the smoothed measurement  C  with controlled standard deviation. The averaging duration applied first by the module  14 A is tailored to be shorter than the duration T chosen as a function of the speed. This first duration is for example of the order of a few tens to a few hundreds of milliseconds. 
     It will be noted that the illustrations of  FIGS. 4 to 6  are merely possible examples of the implementation of the invention. Other configurations may be envisaged. In particular, the speed estimate v could be obtained by measurement means which are different from those described, or even independent of the received radio signals. 
     Moreover, the invention is not limited to the UMTS systems considered hereinabove by way of illustration. It applies for example to second-generation systems, of GSM type or the like, where the radio link supervision also utilizes energy measurements made on the received signals.