Abstract:
A method for estimation of an impulse response (h) of a signal transmission channel ( 1 ) in a mobile communication system and a mobile station ( 15 ), especially a mobile phone, including a device for estimating the impulse response (h) of the signal transmission channel ( 1 ) in the mobile communication system are described. The invention allows a realization of mobile stations for use in a code division multiple access (CDMA) transmission mode and in a time division multiple access (TDMA) transmission mode with less implementation complexity. The estimation device includes exactly one combined channel estimator ( 45 ). The mobile station ( 15 ) receives signals ( 20 ) transmitted via the signal transmission channel ( 1 ) either in the TDMA transmission mode or in the CDMA transmission mode. The exactly one combined channel estimator ( 45 ) estimates the impulse response (h) of the signal transmission channel ( 1 ) according to the transmission mode.

Description:
BACKGROUND OF THE INVENTION 
     The invention proceeds from a method for estimation of an impulse response of a signal transmission channel and a mobile station in accordance with the generic class of the independent patent claims. 
     According to the not yet published German patent application DE 198 26 036 it is already known to suppress distortions of a radio signal caused by multipath reception. Thereby, especially at a transmission of a radio signal in a code division multiple access (CDMA) transmission mode from a base station to a mobile station, an impulse response of a corresponding transmission channel for the multipath reception is estimated. 
     SUMMARY OF THE INVENTION 
     The method for estimation of an impulse response of a signal transmission channel and the mobile station having the characterizing features of the independent patent claims have the advantage that the complexity of the mobile station can be reduced, since there only needs to be one combined channel estimator and one channel estimation routine for the code division multiple access (CDMA) transmission mode and the time division multiple access (TDMA) transmission mode. Therefore, the implementation of a dual mode mobile station which can be used for the CDMA transmission mode as well as for the TDMA transmission mode is simplified. 
     The features of the dependent patent claims enable further improvement of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     An example of the invention is shown in the figures and explained in greater detail in the description below. 
     FIG. 1 is a block diagram of a mobile station according to the invention; 
     FIG. 2 is a flow chart of an algorithm for the method for estimation of the impulse response of the transmission channel according to the transmission mode, 
     FIG. 3 shows the transmission of a signal in the TDMA transmission mode, and 
     FIG. 4 shows the transmission of a signal in the CDMA transmission mode. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1 designates  15  a mobile station which may be realized as a mobile phone, a radio phone or the like. The mobile station  15  comprises an antenna  50  which is connected to a filter  55 . Via the antenna  50 , the mobile station  15  receives signals  20  which are transmitted from a base station,not shown in FIG. 1 via a signal transmission channel  1  in a mobile communication system. According to FIG. 1, two different geographic ways for transmission of the signals  20  to the mobile station  15  are indicated by different arrows. This multipath reception of the signals  20  at the mobile station  15  will cause multipath distortion of the received signals  20 . The filter  55  comprises filter and mixer circuits and transforms the signals  20  into a base-band signal r. The base-band signal  5   r  is then input into a first equalizer  60 , a second equalizer  65  and exactly one combined channel estimator  45  of the mobile station  15 . A first signature reference signal  75  may be provided to the combined channel estimator  45  as well as a second signature reference signal  80 . The exactly one combined channel estimator  45  of the mobile station  15  outputs an estimation for an impulse response h of the signal transmission channel  1  and provides it either for the first equalizer  60  or the second equalizer  65 . An output of the first equalizer  60  and an output of the second equalizer  65  are connected to a multiplexer  70  which multiplexes the output signals of the first equalizer  60  and the second equalizer  65  to produce a multiplexed output signal d which can be decided by a decision device of the mobile station  15 , whereby the decision device is not shown in FIG.  1 . Further devices of the mobile station  15  for processing of the decided multiplexed output signal are also not shown in FIG.  1 . 
     The mobile station  15  is usable for signal transmission in a code division multiple access (CDMA) transmission mode as well as in a time division multiple access (TDMA) transmission mode. The transmission mode utilized for signal transmission between the base station and the mobile station  15  is communicated to the mobile station  15  by the base station at the beginning of the signal transmission. A control unit of the mobile station  15  which is not shown in FIG. 1 activates the first equalizer  60 , if signal transmission in the TDMA transmission mode is provided. If signal transmission in the CDMA transmission mode is provided, the control unit activates the second equalizer  65 . 
     In order to compensate for the multipath distortion of the received signals  20  at the mobile station  15 , it is necessary to know the impulse response h of the signal transmission channel  1 . An estimation of the impulse response h is normally accomplished using so-called signature bits  5  which are multiplexed into the transmitted signals  20 . Another possibility to estimate the impulse response h of the signal transmission channel  1  consists in using blind or adaptive algorithms. 
     In the following, an example is described for the estimation of the impulse response h using the signature bits  5 . In the TDMA transmission mode, the signal  20  transmitted to the mobile station  15  is divided in time slots  35 ,  40  according to FIG.  3 . In FIG. 3, a first time slot  35  and a second time slot  40  are shown as an example on a time axis t. Each time slot  35 ,  40  comprises two information bit sections  30  with information bits  10 . The signature bits  5  are transmitted as a signature bit sequence  25  multiplexed or embedded between the two information bit sections  30  of the corresponding time slots  35 ,  40 . As the timing of the time slots  35 ,  40  and the length of the information bits sections  30  and the signature bit sequences  25  are predetermined and known at the mobile station  15 , the timing of the signature bit sequences  25  in the corresponding time slots  35 ,  40  according to the TDMA transmission mode is also known at the mobile station  15  and especially at the combined channel estimator  45 . 
     In the CDMA transmission mode, the information bits  10  and the signature bits  5  can be code multiplexed and/or time multiplexed. An example for a time multiplexed signal  20  in the CDMA transmission mode is shown in FIG.  4 . According to FIG. 4 the signature bit sequences  25  with the signature bits  5  and the information bit sections  30  with  5  the information bits  10  are multiplexed in a way that each information bit section  30  is embedded into two signature bit sequences  25  and each signature bit sequence  25  is embedded between two information bit sections  30 . Thereby the information bit sections  30  and the signature bit sequences  25  are arranged according to FIG. 4 on the time axis t with predetermined lengths which are known at the mobile station  15 . Therefore, the timing of the signature bit sequences  25  in the CDMA transmission mode is also known at the mobile station  15  and especially in the combined channel estimator  45 . 
     As the timing of the signature bit sequences  25  is known in the combined channel estimator  45  for the TDMA transmission mode as well as for the CDMA transmission mode, the signature bit sequences  25  can be extracted out of the signals  20  received from the antenna  50  and transformed into the base-band by the filter  55 , whereby the actual transmission mode for the transmission of signals  20  between the base station and the mobile station  15  is known in the combined channel estimator  45  from the control unit of the mobile station  15 . Therefore, in the TDMA transmission mode, the signature bits  5  are extracted out of the transmitted signals  20  by the combined channel estimator  45  dependent on their timing in the corresponding time slots  35 ,  40  shown for example in FIG.  3 . In the CDMA transmission mode the signature bits  5  are extracted out of the transmitted signals  20  by the combined channel estimator  45  dependent on their timing shown for example in FIG.  4 . 
     Additionally or alternatively, in the CDMA transmission mode the signature bits  5  may be transmitted code multiplexed with the information bits  10  in the transmitted signals  20 . The coding of the signature bits  5  for  5  transmission in the CDMA transmission mode is also predetermined and known at the mobile station  15  especially in the combined channel estimator  45 , so that the signature bits  5  may also be extracted out of the transmitted signals  20  by the combined channel estimator  45  dependent on their coding in the CDMA transmission mode. The control unit of the mobile station  15  communicates especially at the beginning of signal transmission between the base station and the mobile station  15  the utilized transmission mode to the combined channel estimator  45  such that the combined channel estimator  45  can use the correct timing and/or coding for the extraction of the signature bits  5  out of the transmitted signals  20 . In case of the use of the CDMA transmission mode, the control unit has to communicate to the combined channel estimator  45  also the kind of the CDMA transmission mode, that means if the signature bits  5  are transmitted time multiplexed and/or code multiplexed with the information bits  10 . In this way, the timing and/or coding of the signature bits  5  in the transmitted signals  20  is known in the combined channel estimator  45  for the actually utilized CDMA transmission mode. 
     In case of the use of the TDMA transmission mode, the first signature reference signal  75  is provided to the combined channel estimator  45  and the first equalizer  60  is activated. In case of the use of the CDMA transmission mode, the second signature reference signal  80  is provided to the combined channel estimator  45  and the second equalizer  65  is activated. The combined channel estimator  45  compares the signature bit sequences  25  extracted out of the transmitted signal  20  with the corresponding signature reference signal  75 ,  80  of the actually utilized transmission mode. Dependent on the comparison result, the combined channel estimator  45  estimates the impulse response h of the signal transmission channel  1  and provides the estimation to the activated equalizer  60 ,  65 . 
     According to the estimation of the impulse response h of the signal transmission channel  1  an impulse response e is calculated and realized for the activated equalizer  60 ,  65  such that h * e≈1. In this way, the influence of the signal transmission channel  1  which is the multipath distortion may be approximately compensated by the activated equalizer  60 ,  65  such that the output of the activated equalizer  60 ,  65  approximately corresponds to the information bits  10  originally sent by the base station. 
     If an adaptive algorithm is used for the estimation of the impulse response h of the signal transmission channel  1 , the combined channel estimator  45  may be initialized by an extraction of signature bit sequences  25  at the beginning of the signal transmission between the base station and the mobile station  15 . That means, that in the way described above, a first estimation for the impulse response h of the signal transmission channel  1  may be calculated during a training period. Afterwards previously received information bits  10  which were detected and equalized by the activated equalizer  60 ,  65  according to the first estimation and decided by the decision device not shown in FIG. 1 may be used by the combined channel estimator  45  to adapt the estimation of the impulse response h of the signal transmission channel  1  in order to track relatively slow variations in signal transmission channel characteristics during a so called decision-directed mode. The adaptation of the estimation of the impulse response h of the signal transmission channel  1  during the decision-directed mode is realized especially after completion of the training period. 
     The adaptive algorithm described above may be applied for every time slot  35 ,  40  in the TDMA transmission mode. Thereby, the estimation of the impulse response h of the signal transmission channel  1  is first initialized by the combined channel estimator  45  using the signature bit sequence  25  in every time slot  35 ,  40  in the above-described manner during the training period. The information bits  10  before and after the signature bit sequence  25  in the corresponding time slot  35 ,  40  are saved in a memory of the mobile station  15  not shown in FIG. 1. A first one of the information bits  10  of the corresponding time slot  35 ,  40  will then be detected an equalized based upon the initialized estimation of the impulse response h of the signal transmission channel  1 . All further information bits  10  of the corresponding time slot  35 ,  40  will then be detected and equalized by the activated first equalizer  60  based upon an adaptive estimation of the impulse response h of the signal transmission channel  1  taking account of previously detected, equalized and decided information bits  10  of the same time slot  35 ,  40  during the decision-directed mode. 
     Another possibility for the estimation of the impulse response h of the signal transmission channel  1  by the combined channel estimator  45  consists in utilizing a blind algorithm. Thereby, no signature reference signal  75 ,  80  is provided to the combined channel estimator  45  or signature bit sequences  25  do not exist in the transmitted signals  20 . 
     FIG. 2 shows an algorithm for the estimation of the impulse response h of the signal transmission channel  1  during the training period. At a step  100 , the control unit of the mobile station  15  checks if the TDMA transmission mode or the CDMA transmission mode is selected for signal transmission between the base station and the mobile  5  station  15 . If the TDMA transmission mode is selected, the program branches to a step  105 , otherwise it branches to a step  110 . At step  105 , the combined channel estimator  45  extracts the signature bit sequences  25  out of the transmission signal  20  dependent on the timing of the signature bits  5  in the corresponding time slots  35 ,  40 . Afterwards the program branches to a step  115 . At step  115 , the first signature reference signal  75  is loaded into the combined channel estimator  45 . At step  110 , the combined channel estimator  45  extracts the signature bit sequences  25  out of the transmitted signals  20  dependent on the timing and/or coding of the signature bits  5  in the transmitted signals  20 . Afterwards the program branches to a step  120 . At step  120 , the second signature reference signal  80  is loaded into the combined channel estimator  45 . After step  115  and after step  120 , the program branches to a step  125 . At step  125 , the combined channel estimator  45  compares the loaded signature reference signal  75 ,  80  with the extracted signature bits  5 . Afterwards, the program branches to a step  130 . At step  130 , the combined channel estimator  45  calculates an estimate for the impulse response h of the signal transmission channel  1 . Afterwards the program is left. 
     The comparison of the corresponding signature reference signal  75 ,  80  with the extracted signature bits  5  and the calculation of the estimation for the impulse response h of the signal transmission channel  1  may be realized utilizing known algorithms as for example a least mean square algorithm as described in “Communication Systems”, 2 nd  edition, Simon Haykin, John Wiley &amp; Sons, New York, 1983, a Kalman estimator as described in “Adaptive Filter Theory”, 2 nd  edition, Simon Haykin, Prentice Hall, 1991, a correlator estimator or the like. 
     As it concerns the CDMA transmission mode, the second equalizer  65  may be realized as a RAKE receiver as described in “Digital Communications”, 3rd edition, John G. Proakis, McGraw-Hill, New York, 1995.