Abstract:
There is proposed a technique for determining parameters, especially channel estimation, in the receiver of a communication system utilizing adaptive antenna techniques. The technique uses information transmitted in the antenna signals and in the beam signals to determine the parameters.

Description:
This application is a continuation of U.S. patent application Ser. No. 10/465,995, filed Nov. 24, 2003, which is a 371 Application of International Application No. PCT/EP01/15152, filed Dec. 20, 2001. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a technique for estimating parameters in the receiver of an adaptive antenna system, and particularly but not exclusively to channel estimation in the receiver of a mobile station in a mobile communication system. 
     BACKGROUND TO THE INVENTION 
     In wide-band code division multiple access (W-CDMA) systems, one of the most demanding tasks having a significant effect on the performance of the receiver is the channel estimation, in which the complex channel coefficients are estimated. The goal of channel estimation is to cancel the distortion caused to the transmitted signal by the radio channel such that as perfect a replica of the transmitted signal is retrieved as is possible. 
     This can be achieved by coherent demodulation in which the received signal is multiplied with the complex conjugated channel estimate. However, in multi-path fast fading conditions the coherent detection is difficult to achieve and a channel estimation method is required that works satisfactorily under the challenging fast fading circumstances. 
     The channel multi-path profile heavily affects the performance of the channel estimation due to the fact that the estimation must be done individually in each temporal rake finger of the rake receiver. The more multi-path components in the channel, the lower the signal-to-noise ratio (SNR) per rake finger, since the channel energy is distributed in the propagation paths. 
     In the forward link, i.e. the down-link, of a wide-band code division multiple access (WCDMA) system, a primary common pilot channel (P-CPICH) is broadcast over the entire cell or a sector. The P-CPICH is broadcast also in the case of a multi-beam arrangement (multiple beams per sector) and in user specific beam-forming. Therefore there always exists one such channel per sector regardless of the applied transmission scheme. 
     In systems utilizing adaptive antenna techniques, dedicated channels are usually transmitted through a narrow beam, which means that the P-CPICH and the down-link dedicated physical channels (DL-DPCH) usually experience different channel characteristics on transmission to a mobile station antenna. 
     In current known systems, the down-link dedicated physical control channel (DL-DPCCH) is used as the phase reference for the DL-DPCH in adaptive antenna systems because of the fact that the P-CPICH does not experience the same channel characteristics. 
     It is therefore an aim of the present invention to provide an improved technique for estimating parameters in the receiver of an adaptive antenna system. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, a disclosed method includes receiving a beam signal at a receiver of a communication system, the receiver having an adaptive antenna transmitter. The communication system is a cellular system including a plurality of cells, each cell including at least one sector. An antenna signal is received at the receiver. Furthermore, parameters of the received beam signal are estimated based on information received in the received antenna signal. Specifically, the antenna signal is transmitted over an entire sector and the beam signal is transmitted over part of a sector and the estimating of the parameters is further based on the received beam signal. 
     In one aspect of the present invention, a disclosed method includes receiving a beam signal having a dedicated channel at a receiver, the receiver having an adaptive antenna transmitter. The communication system is a cellular system including a plurality of cells, each cell including at least one sector. An antenna signal is received at the receiver and the antenna signal has a primary common pilot channel. Channel estimation is performed on the received beam signal based on pilot signals received in the primary common pilot channel. The antenna signal is transmitted over an entire sector and the beam signal is transmitted over part of a sector and the channel estimation is additionally based on dedicated signals received in the dedicated physical channel. 
     In another aspect of the present invention, a disclosed apparatus includes an adaptive antenna transmitter and a first input element configured to receive a beam signal. A second input element in the apparatus is configured to receive an antenna signal. An estimator in the apparatus is connected to the second receiver and is configured to estimate parameters of the received beam signal based on information received in the antenna signal. The antenna signal is transmitted in a communications system that includes multiple cells and each of the cells includes at least one sector. The antenna signal is transmitted over an entire sector, and the beam signal is transmitted over part of a sector. The estimator is further connected to the first input element and is configured to estimate the parameters based on information additionally received in the beam signal. 
     In another aspect of the present invention, a disclosed method includes receiving a beam signal at a receiver of a communications system, where this receiver includes an adaptive antenna transmitter. An antenna signal is also received at the receiver, and parameters of the received beam signal are estimated based on information received in the received antenna signal. The antenna signal includes a common pilot channel. Also, the estimating of the parameters is further based on the received beam signal and on signals received in the common pilot channel. The method is preferably operable in a cellular communication system that includes multiple cells, such that each cell includes at least one sector. The antenna signal is transmitted over an entire sector and the beam signal is transmitted over a part of a sector. 
     In one aspect of the present invention, a disclosed method includes receiving a beam signal in a receiver of a communications system. The receiver includes an adaptive antenna transmitter, and the communication system is a cellular communication system that includes multiple cells, each having at least one sector. An antenna signal is received at the receiver. Parameters of the received beam signal are estimated based on information received in the received antenna signal/Also, the antenna signal is transmitted over an entire sector and the beam signal is transmitted over a part of a sector, such that the estimating of the parameters is further based on the received beam signal. The estimating of the parameters also includes estimating a channel of the beam signal, calculated by exploiting a statistical correlation property between the beam signal and antenna signal. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention will be best understood by way of example with reference to the following Figures in which: 
         FIG. 1  illustrates exemplary W-CDMA base station cells utilizing different transmission schemes in each cell (three-sectorised configuration); and 
         FIG. 2  illustrates in block diagram form elements of a receiver necessary for implementing the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to  FIG. 1 , there is now described examples of multi-sector W-CDMA cells with respect to which the invention is illustrated. The invention is not, however, in any way limited to such a specific example. 
     A plurality of mobile stations, or user equipment roam within the cell. For example, as shown in  FIG. 1 , mobile station  130  is connected in cell  106 , mobile station  132  is connected in cell  104 , and mobile station  134  is connected in cells  104  and  108 . 
     The base station cell  102  is divided into N sectors, where N=3 in the example of  FIG. 1 . 
     As exemplified by sector  106  of  FIG. 1 , each sector can be divided into either K fixed beams or steerable (user specific) beams using a base transceiver station  112 . The beams  116  represent the secondary common pilot channel, the beam  120  the down-link dedicated physical channel, and the beam  118  the primary common pilot channel. 
     Sector  104  of  FIG. 1  illustrates the traditional single antenna transmission scheme utilizing a base transceiver station  110 . The beam  120  the down-link dedicated physical channel, and the beam  118  the primary common pilot channel. 
     Sector  108  of  FIG. 1  illustrates user specific beam-forming using a base transceiver station  114 . The beam  120  the down-link dedicated physical channel, and the beam  118  the primary common pilot channel. 
       FIG. 1  thus illustrates the CPICHs needed in the different transmission schemes, and the DL-CPCH of a single user. 
     For the purposes of describing the present invention, two of the three base transceiver stations of the example of  FIG. 1  use adaptive antenna techniques for communicating with mobile stations in the various sectors of the cell. The two cells utilizing adaptive antenna techniques are cells  106  and  108 . Adaptive antenna techniques are well-known in the art, and the present invention is not directly concerned with any specific implementation details of such techniques. As a skilled person will be familiar with, when using adaptive antenna techniques the base transceiver station  100  transmits mobile specific data to a mobile station through a narrow beam. 
     The W-CDMA specification defines three different types of pilot channels in the forward link for an adaptive antenna system. These pilot channels are: 1. P-CPICH (Primary Common Pilot Channel); 2. S-CPICH (Secondary Common Pilot Channel); and 3. Dedicated pilot symbols in DPCCH (Dedicated Physical Control Channel). 
     The P-CPICH is broadcast over an entire sector in a multi-sector arrangement, and there exists only one such channel for each sector. The P-CPICH is used in the hand-over measurements and cell selection/reselection procedures. Another function of the P-CPICH channel, when the common channels are not associated with dedicated channels or not involved in adaptive antenna techniques, is to aid the channel estimation at the mobile station for the dedicated channels, and to provide a channel estimation reference for the common channels. 
     The S-CPICH may be transmitted over the entire cell or over only part of the cell. There may be zero, one or several S-CPICHs per cell or sector. One typical area of S-CPICH usage is operations with base stations having multiple (fixed) beams per sector. The S-CPICHs are used for identifying different beams at the mobile station. 
     The dedicated pilot symbols are multiplexed into the down-link dedicated physical channel (DPCH). They are used in signal-to-interference ratio (SIR) estimation and are also used in the channel estimation. If the mobile station or user equipment is informed that the P-CPICH is not the phase reference and there is no S-CPICH available, then the dedicated pilot bits in the DL-DPCCH are the phase reference for the DL-DPCH. This may happen, for example, in the case of user-specific beam forming. 
     In accordance with the present invention, it is proposed to use the primary common pilot channel P-CPICH for estimating parameters in a mobile station or user equipment of the adaptive antenna system. Preferably, as described in further detail hereinbelow, the primary common pilot channel is used in combination with the existing channels for estimating parameters. Particularly advantageously, the primary common pilot channel is used in channel estimation in the mobile station. 
     Even though user specific beam forming is applied in adaptive antenna systems, the P-CPICH must be broadcast. This means that there is a strong-powered pilot channel that is available to all mobile stations. In many cases, the SNR of the continuous and non-power-controlled P-CPICH is much better than that of time-multiplexed and power-controlled DL-DPCCH. The relative difference of SNRs (P-CPICH vs. DL-DPCH) gets biggest when the mobile station is situated near to the base station. 
     A proposed implementation of a channel estimation technique in accordance with the present invention is provided hereinafter. 
     In the general case the joint channel estimation scheme is preferably designed to be adaptive since the channel characteristics change as a function of time. The adaptivity of the joint channel estimator could be based for example on the correlation measurement. 
     The following equations explain one possible way to implement the joint channel estimator. The joint solution is calculated as #joint=##DL-DPCCH+(1-#)#P-CPIC (1) in which hP-CPICH and hDL-DPCCH are the channel estimates obtained from the P-CPICH and DL-DPCCH and β is the weight factor. The weight factor can be calculated as in which the expectation value of the time variant correlation coefficient, /3, is calculated as Referring to  FIG. 2 , there is illustrated a block diagram of the main elements of a receiver suitable for implementing the present invention in an exemplary W-CDMA system. Referring to  FIG. 2 , there is provided three correlator banks  202 , 204  and  206 . 
     There is further provided a detection and rake combiner  214 , and a channel decoding block  218 . 
     The correlator-bank  202  acts as an input means to the receiver for normal data transmission. The correlator bank  204  acts as an input means to the receiver for the dedicated pilot channels in the beam signal from the transmit antenna. The correlator bank  206  acts as an input means to the receiver for the broadcast channels in the antenna signal from the transmit antenna. 
     The three correlator banks perform the correlation for the respective signals, all of which are received at the receiver antenna as generally indicated by line  200 . 
     Each of the outputs of the correlators  202 ,  204 , and  206  produces a respective output on lines  208 , 210  and  212 , which form inputs to the detection and rake combiner  214 . The output of the rake combiner  214  on line  216  is provided to the channel decoding block. 
     The channel decoded signal is then provided on line  220  for further processing. 
     There can be a significant correlation between the beam signal and the antenna signal in the case of user specific beam forming. Correlation between the P-CPICH and DL-DPCCH can be increased if the P-CPICH is transmitted via all of the antenna elements (instead of single antenna only). The narrower the angular spread (seen from the base transceiver station) the more correlated the antenna and beam signals are. The correlation property can be seen from the fact that P-CPICH based channel estimation (antenna signal) performs well (depending on the angular spread) even though the DL-DPCHs are transmitted through the beam signal. The correlation property can be exploited by using both P-CPICH and DL-DPCCH in the channel estimation of DL-DPCH (joint channel estimation). 
     If the angular spread in the radio channel is small and the SNR per tap low (noise limited environment) the P-CPICH based scheme performs better than DL-DPCCH based channel estimation. However, at high SNR values the dedicated pilot based scheme performs better than the P-CPICH based scheme. The performance of channel estimation can thus be improved even by selection combining (compared to P-CPICH and DL-DPCCH only based schemes). On the other hand, if both P-CPICH and DL-DPCCH are used in the channel estimation of DL-DPCH (joint channel estimation) all energy of the transmitted pilot signal could be utilized. Thus the joint estimation is expected to always be better than either P-CPICH nor DL-DPCCH only based schemes. 
     The S-CPICH can also be used to aid the channel estimation at a mobile station for the dedicated channels, for example, in the case of multi-beam transmission. Thus, the principle of joint parameter estimation could be applied also in the case of S-CPICH transmission (S-CPICH &amp; P-CPICH, S-CPICH &amp;amp; P-CPICH &amp; DL-DPCH). 
     In a macro-cellular radio environment it is assumed that: the angular spread is typically relatively low; there are multiple channel taps (SNR per channel tap is low, noise limited area from the parameter estimation point of view); multiple channel taps (each tap is a separate cluster in the angular domain); LOS (strong correlation, narrow angular spread); and the speed of mobile can be high. 
     The present invention thus provides a technique in which the correlation between the beam signal (DL-DPCH) and the antenna signal (P-CPICH) in the parameter estimation (especially channel estimation) is used. This correlation is advantageously exploited in a joint channel estimation scheme, which is preferably adaptive since the correlation properties change as a function of time. The adaptivity of the joint channel estimator may be based, for example, on the correlation measurement.