Abstract:
A base station forming part of a cellular communications system includes a reference signal generator, a boost selector and a transmitter. The reference signal generator provides a dedicated reference signal to be transmitted over plural antennas to a dedicated user device. The boost selector selects a dedicated boost level specific for the dedicated reference signal and the transmitter transmits the dedicated reference signal boosted by its dedicated boost level to the user device. A user equipment device includes a receiver, a boost determiner and a data demodulator. The receiver receives a dedicated reference signal transmitted by a base station where the dedicated reference signal has been boosted by a dedicated boost level specific to the user device. The boost determiner determines the dedicated boost level and the data demodulator demodulates a data signal from the base station using the dedicated boost level and a channel estimation value.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit from U.S. Provisional Patent Application No. 61/040,486, filed Mar. 28, 2008, which is hereby incorporated in its entirety by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to wireless communication, and in particular to channel estimation in cellular networks. 
       BACKGROUND 
       [0003]    In cellular communications systems, a multiplicity of base stations may be distributed throughout a geographical area. Each base station communicates with user equipment (UE) devices, such as cellular phones, for example, which may move throughout the geographical area. 
         [0004]    In order for the UE to perform coherent demodulation of the signals that it receives from a base station with which it is currently communicating, the UE needs to perform channel estimation. In order to facilitate channel estimation at the UE side, the base station may periodically broadcast, at known times and/or at known frequencies, “pilot” signals which are known to the UE and may be utilized to acquire the necessary channel information. Pilot signals may also be referred to as “reference signals” (RS), and these terms are used interchangeably throughout the following disclosure. 
       SUMMARY 
       [0005]    There is provided, in accordance with an embodiment of the present disclosure, a base station forming part of a cellular communications system. The base station includes a reference signal generator, a boost selector and a transmitter. The reference signal generator provides a dedicated reference signal to be transmitted over plural antennas to a dedicated user device in a cellular reception area. The boost selector selects a dedicated boost level that is specific for the dedicated reference signal and the transmitter transmits the dedicated reference signal boosted by its dedicated boost level to the user device. In accordance with an embodiment of the disclosure, a first dedicated boost level applied to a first dedicated reference signal transmitted to a first dedicated user may be different than a second dedicated boost level applied to a second dedicated reference signal that is transmitted to a second dedicated user. 
         [0006]    Moreover, in accordance with an embodiment, the boost selector includes a boost level setter to set the boost level to generate a predetermined demodulation penalty when a transmitted signal is demodulated by the dedicated user device. 
         [0007]    Further, in accordance with an embodiment, the demodulation penalty is a function of a channel estimation processing gain plus the boost level. 
         [0008]    Alternatively, in accordance with an embodiment, the boost selector includes a boost level setter to predefine a set of boost levels associated with modulation and coding scheme values and to select a boost level based on a currently selected modulation and coding scheme. 
         [0009]    Further alternatively, in accordance with an embodiment, the boost selector includes a boost level setter to predefine a set of boost levels and to select an appropriate boost level. 
         [0010]    Moreover, in accordance with an embodiment, the boost level is a downboost level. 
         [0011]    Additionally, in accordance with an embodiment, the cellular communications system is an LTE communications system. The boost level changes the power level of the dedicated reference signal with respect to a transmitted data signal. 
         [0012]    There is provided, in accordance with an embodiment of the present disclosure, a method for a base station. The method includes providing a dedicated reference signal to be transmitted over plural antennas to a dedicated user device in a cellular reception area, selecting a dedicated boost level that is specific for the dedicated reference signal and transmitting the dedicated reference signal boosted by its dedicated boost level to the user device. 
         [0013]    There is also provided, in accordance with an embodiment of the present disclosure, method for a user equipment device of n cellular communications system. The method includes receiving a dedicated reference signal transmitted by a base station where the dedicated reference signal has been boosted by a dedicated boost level specific to the user device, determining the dedicated boost level and demodulating a data signal from the base station using the dedicated boost level and a channel estimation value. 
         [0014]    Further, in accordance with an embodiment, the determining includes estimating a channel estimate from the reference signal and inferring the boost level from a predefined demodulation penalty which is a function of a gain produced by the channel estimate plus the boost level. 
         [0015]    Alternatively, in accordance with an embodiment, the determining includes receiving a modulation and coding scheme value and setting the boost level to a boost level associated with the received modulation and coding scheme value. 
         [0016]    Further alternatively, in accordance with an embodiment, the determining includes estimating the boost level as a function of a measured power ratio between received data signals and reference signals and selecting the boost level to be a boost level, from among a set of predefined boost levels, whose value is closest to the magnitude of the measured power ratio. 
         [0017]    Still further, in accordance with an embodiment, the determining includes providing each of a set of predefined boost levels to a data demodulator and selecting the boost level which minimizes demodulation errors of the data demodulator. 
         [0018]    There is also provided, in accordance with an embodiment of the present disclosure, a user equipment device forming part of a cellular communications system. The device includes a receiver, a boost determiner and a data demodulator. The receiver receives a dedicated reference signal transmitted by a base station where the dedicated reference signal has been boosted by a dedicated boost level specific to the user device. The boost determiner determines the dedicated boost level and the data demodulator demodulates a data signal from the base station using the dedicated boost level and a channel estimation value. 
         [0019]    Moreover, in accordance with an embodiment, the determiner includes an estimator to estimate a channel estimate from the reference signal and an analyzer to infer the boost level from a predefined demodulation penalty which is a function of a gain produced by the channel estimate plus the boost level. 
         [0020]    Further, in accordance with an embodiment, the determiner includes a receiver to receive a modulation and coding scheme value and a boost level setter to set the boost level to a boost level associated with the received modulation and coding scheme value. 
         [0021]    Alternatively, in accordance with an embodiment, the determiner includes an estimator to estimate the boost level as a function of a measured power ratio between received data signals and reference signals and a boost selector to select the boost level to be a boost level, from among a set of predefined boost levels, whose value is closest to the magnitude of the measured power ratio. 
         [0022]    Further alternatively, in accordance with an embodiment, the determiner includes a unit to provide each of a set of predefined boost levels to the data demodulator and a selector to select the boost level which minimizes demodulation errors of the data demodulator. 
         [0023]    Still further, in accordance with an embodiment, the boost level is a downboost level. 
         [0024]    Finally, in accordance with an embodiment, the cellular communications system is an LTE communications system and the boost level changes the power level of the dedicated reference signal with respect to a transmitted data signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The subject matter of various embodiments, may best be understood with reference to the following figures, in which: 
           [0026]      FIG. 1  is a schematic illustration of a base station configured to steer a transmitted beam towards a UE; 
           [0027]      FIG. 2  is a schematic illustration of a base station configured to steer transmitted beams including data symbols and reference symbols toward different users in accordance with an embodiment of the invention; and 
           [0028]      FIG. 3  is a schematic illustration of elements of a user equipment device, constructed and operative in accordance with a preferred embodiment, for demodulating the signals from the base station of  FIG. 2 . 
       
    
    
       [0029]    It is noted that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements. Further, where considered appropriate, similar reference numerals that are repeated among the figures indicate corresponding or analogous elements. 
       DETAILED DESCRIPTION 
       [0030]    The 3rd Generation Partnership Project (3GPP) is presently defining various communication protocols for the emerging next generation, LTE (Long Term Evolution) Advanced cellular telecommunication standard, which employs an air interface formally referred to as Evolved-UMTS Terrestrial-Radio Access (E-UTRA). 
         [0031]    Reference is now made to  FIG. 1 , which illustrates a base station  10  transmitting signals along a user channel  12  to a user equipment device (UE)  14 . In accordance with emerging LTE standards, the base station  10  may employ “beamforming” techniques to steer a transmitted signal along user channel  12  so that it may be best received by the intended UE ( 14 ). With beamforming, a user signal S user  is transmitted through a multiplicity of antennas  16  (three antennas are shown in this example, although a different number of antennas may be used), where each copy of the signal is multiplied by a different complex valued weight w i  (i.e. a gain factor and a phase offset) for each antenna. The combination of the weights define a direction to the beam and/or a region of best reception. 
         [0032]    The common reference signals of LTE, which are common to all UEs serviced by a base station, are generally inappropriate for channel estimation by a UE  14  receiving its data signal through beamforming, because the common reference signals are not transmitted through any beamforming weights while the UE&#39;s data signal is. Thus, for the purpose of coherent data demodulation for UEs in beamforming mode, these UEs have dedicated reference signals. The dedicated reference signal for each UE undergoes exactly the same beamforming weights (and consequently, the same channel conditions) as its UE&#39;s data signal, and are therefore used for the construction of the channel estimator for the beamforming mode. 
         [0033]    In accordance with the emerging LTE-A standards, UEs operating in beamforming mode, such as UE  14 , may also receive dedicated reference signals.  FIG. 2 , to which reference is now made, illustrates multiple reference signals RS transmitted through multiple antennas  16 , for two exemplary users, user 1  and user 2 . Different weights may be used for different UEs; thus,  FIG. 2  shows a reference signal RS user1  and a data signal S user1  undergoing one set of weights optimized for user 1 , and a reference signal RS user2  and data signal S user2  undergoing another set of weights optimized for user 2 . Both the data signals S and the reference signals RS are transmitted through the same beamforming antennas  16 ; however, since user 1  and user 2  are in different locations, the weights multiplying the input signals (either the data signal S or the reference signal RS) may be different. Accordingly,  FIG. 2  shows the two separately. In some cases, user 1  and user 2  may be served by different portions of the time-frequency domain, and in other cases (often known as multi-user MIMO) they may share, or partially share, the time-frequency resources. 
         [0034]    When the user devices UEs  14  may be in particularly noisy locations and thus, channel  12  of  FIG. 1  may have a high level of noise, UEs  14  may only be able to generate a poor quality channel estimation from its dedicated reference signals. This will degrade the quality of the data demodulation. In such a case, it may be desirable to “boost” (i.e. increase) the power of the reference signals. This may increase the signal to noise ratio of the reference signals, thereby enabling UEs  14  to calculate a more accurate channel estimate and ultimately resulting in an improved data demodulation. In another embodiment, it may make sense to reduce the power (i.e. “downboosting”) to the dedicated reference signals, e.g. for the purpose of reducing the interference to other UEs in the cellular network. It is noted that, in the LTE draft specification R1-081108: “Way Forward on Dedicated Reference Signal Design for LTE downlink with Normal CP”, Sorrento, February 2008, neither boosting nor downboosting reference signals is contemplated. 
         [0035]    In general, the boosting/downboosting may change the power level of the transmitted signal. The boost typically may be with respect to the power level in the data signal to be demodulated. Alternatively, it may be with respect to some other suitable, predefined power level. This power level may be a fixed power level, fixed by the system, or it may be a changeable power level, changed for a dedicated user device. In which case, the boost level may be the extra power over the changed power level. 
         [0036]    In accordance with an embodiment, base station  10  may comprise a boost selector  20  which may select boost/downboost values G for each user. Base station  10  may multiply the relevant reference signal RS by the relevant boost value G (G user1  for RS user1  or G user2  for RS user2 ) and may transmit the resultant signal through the antennas  16 , after multiplication by the appropriate beamforming weights w. 
         [0037]    The UE typically needs to know the strength of the RS boost, in order to correctly scale the channel estimator to match the data signal (since the RS boost may add a scale factor to the output of the estimator). In accordance with an embodiment of the disclosure, base station  10  may signal the magnitude of the dedicated RS boost relative to the data to the UE. In another embodiment, the UE may blindly detect the RS-to-data boost. 
         [0038]    Reference is now made to  FIG. 3 , which illustrates a user equipment device  14 , constructed and operative in accordance with an embodiment of the disclosure, which demodulates data in a beamforming mode with boosted reference signals. 
         [0039]    User equipment device  14  may comprise a receiver  28 , a dedicated channel estimator  30 , a boost determiner  32  and a data demodulator  34 . Receiver  28  may be an antenna or any other device capable of receiving reference signal RS user  and data signal S user . Dedicated channel estimator  30  may be any suitable channel estimator, such as a Weiner estimator described in Chapter 14 of the book OFDM and MC-CDMA for Broadband Multi-User Communications, WLANs and Broadcasting by L. Hanzo et al, John Wiley &amp; Sons, 2003. Dedicated channel estimator  30  may receive the dedicated reference signal RS user  and may generate an initial channel estimate h therefrom. 
         [0040]    Boost determiner  32  may determine the boost value. As described hereinbelow, base station  10  may signal the UE with the boost value, in which case, boost determiner  32  of UE  14  may process the signaling to determine the boost value. In another embodiment, described hereinbelow, boost determiner  32  may blindly estimate the ratio between pilot-to-data power and may determine the boost from this estimate. Finally, data demodulator  34  may utilize channel estimate h (produced by channel estimator  30 ) and boost level G (from boost determiner  32 ) to demodulate any incoming data signals S user . 
         [0041]    In base station  10 , boost selector  20  ( FIG. 2 ) may select the dedicated boost value G user  in any suitable way in order to optimize the overall network performance. 
         [0042]    In one embodiment, boost selector  20  may set the power for the dedicated reference signal (known as the “DPICH” in the LTE standard) so that some agreed upon, fixed, a-priori data demodulation penalty may be encountered, given an agreed upon channel estimator, such as estimator  30 . For example, if the reference boost level is set such that the post processing channel estimation errors are 10 dB lower than the combination of noise and interference at the output of data demodulator  34 , an overall data demodulation penalty of about 0.5 dB will be observed. In general, for this embodiment, boost selector  20  may set the boost level such that the sum of a channel estimation processing gain plus the power boost may yield the desired a-priori known demodulation penalty. Boost determiner  32  (in UE  14 ) may infer the boost level from the known penalty. For example, dedicated channel estimator  30  typically may determine post processing channel estimation errors resulting from noise samples from a plurality of dedicated reference pilots, averaged together. If these errors are different than the expected error level of 10 dB lower than the combination of noise and interference at the output of data demodulator  34 , then the boost level is the cause. For example, if the post processing errors are only 3 dB lower and the expected post processing errors arc targeted at 10 dB below the total noise plus interference level, then the reference signal boost is 7 dB. Or, if the post processing errors are 13 dB lower, then the downboost is −3 dB. 
         [0043]    Another embodiment may utilize the fact that, in the LTE standard, the modulation and coding scheme (MCS) is determined based on SNR reports (called CQI). Since the extent of the boost may also be dependent on the SNR levels and/or the MCS, in this embodiment, boost selector  20  may have a set of predefined reference boost levels, one for each MCS, and may select the boost level based on the currently selected MCS. These boost levels, as a function of MCS, may be determined from downlink performance simulations with the DPICH reference signals. 
         [0044]    Because the MCS is signaled to UEs  14 , boost determiner  32  may determine the boost level as the boost level associated with the current MCS value. 
         [0045]    In another embodiment, boost selector  20  may have a predefined set of boost levels, e.g.; 0, 3 and 6 dB, from which to select. Boost determiner  32  may then estimate the boost level from the measured power ratio between received data signals and reference signals, selecting the boost level to be the predefined boost level whose value is closest to the measured power ratio. 
         [0046]    Alternatively, boost determiner  32  may alternately provide each of the boost levels to data demodulator  34  and may select the boost level which minimizes demodulation errors. 
         [0047]    Unless specifically slated otherwise, as apparent from the disclosure herein, any “processing,” “computing,” “calculating,” “determining,” or similar operations, refer to operations that may be performed in dedicated computing hardware, or in a generalized computer device using firmware or software. 
         [0048]    While certain features of embodiments of the invention have been illustrated and described herein by way of example, many modifications, substitutions, variations, changes, combinations and equivalents may be apparent to those of ordinary skill in the art. Accordingly, the scope of the present invention as embodied in the claims appended hereto is intended to cover all such modifications, substitutions, variations, changes, combinations and equivalents occurring to a person of ordinary skill based on the foregoing description and which are not disclosed in the prior art.