Abstract:
The present invention relates to a method for gaining channel quality information at a base station of a multi-carrier radio communication system having a plurality of orthogonal frequency sub-carriers. According to the present invention, the method comprises the steps of: measuring a Channel Frequency Response at a user terminal of said multi-carrier radio communication channel; extracting samples of said Channel Frequency Response represented in the frequency domain at a predefined oversampling factor; sending said samples on a signaling channel from said user terminal to said base station; performing a windowing operation on the received samples in the time domain; determining a channel quality information for each of said sub-carriers on a representation in the frequency domain of said received samples after windowing operation.

Description:
[0001]     The invention is based on a priority application EP05300922.1 which is hereby incorporated by reference.  
       TECHNICAL FIELD  
       [0002]     The present invention relates to a method for optimizing the use of the feedback channel in multi-carrier systems.  
       BACKGROUND OF THE INVENTION  
       [0003]     Such multi-carrier systems often implement Orthogonal Frequency Division Multiplexing (OFDM) as multi-carrier transmission technique. OFDM is for example used in the HIPERLAN/2 standard as well as an extension of the IEEE 802.11 a standard for the 5 Ghz area. OFDM could also offer a sensible alternative for high-speed mobile applications, and thus represents an important step for next generation mobile radio systems or for a 4 th  generation air interface. To this extend, the 3 rd  Generation Partnership Project (3GPP), for the standardization of high-speed broadband wireless mobile communication systems, is recently considering the application of OFDM techniques for the high speed data packet access (HSDPA) air interface communication between the radio access network (RAN) and the user terminal (UE).  
         [0004]     In multi-carrier systems as OFDM transmission system, the transmitted data is split into a number of parallel data streams, each one used to modulate a separate sub-carrier. In other words, the broadband radio channel is subdivided into a plurality of narrow-band subchannels or sub-carriers being independently modulated with e.g. QPSK, 16 QAM, 64 QAM or higher modulation order allowing higher data rate per sub-carrier.  
         [0005]     In such OFDM systems, the sub-carrier frequencies can be allocated to a user channel on a short term basis (e.g. all 2 ms) as well the modulation order per sub-carrier defining a transmission channel for each user should be updated on the same short term basis.  
         [0006]     A very important task of such multi carrier systems to provide for an efficient sub-carrier/modulation allocation to the different users. This is necessary to optimize and extend the performance of the multi-carrier system.  
         [0007]     On the one hand, the selection of the best appropriate sub-carrier for each user should take into account that in mobile environment with multipath radio channels some sub-carriers may be subject to very strong channel attenuation when seen by a given user. Such sub-carriers would be useless allocated to this user because of fading. On the contrary they may be received with good quality by other users.  
         [0008]     On the other hand, when the best appropriate sub-carriers are identified for a user, the optimal modulation to be used on these sub-carriers should be appropriately selected. The higher modulation orders can only be used if the signal to noise ratio (SNR) at the receiver is high enough to allow the demodulation.  
         [0009]     All this optimization requires exchange of signaling/measure information in a feedback channel, so that the entity of the network which is responsible for resource allocation, respectively for modulation selection (e.g. the base station or Node B) disposes of quality measures performed at the different user equipments (UE).  
         [0010]     In HSDPA (High Speed Downlink Packet Access) systems, only one channel quality measure is transmitted from the user terminal to the base station. This channel quality information CQI is defined in 3GPP standards, 3GPP 25.214 and 3GPP 25.211. The channel quality information is however not accurate enough for multi channel systems  
         [0011]     In the ideal case, each user terminal should make a channel quality measurement for each sub-carrier and report it on the feedback channel to the Node B. The drawback is that this would represent a huge amount of signaling information (some Mbits) especially in multi carrier system with up to thousands of sub-carriers.  
         [0012]     A particular object of the present invention is to provide a method for sending feedback information regarding the channel quality in a more efficient way so that the amount of feedback information is reduced but anyway sufficient for performing an efficient resource allocation in the multi carrier system.  
         [0013]     Another object of the invention is to provide a corresponding user terminal and base station.  
       SUMMARY OF THE INVENTION  
       [0014]     These objects, and others that appear below, are achieved by a method for gaining channel quality information in a multi-carrier radio communication system according to claim  1 , a user terminal according to claim  8 , and a base station according to claim  9 .  
         [0015]     According to the present invention, detailed and reliable channel quality information about the Channel Frequency Response of the radio channel are gathered at the base station. The channel quality information is compressed and sent with redundancy by the user terminal to the base station over the feedback channel. The base station decodes this information, minimizes the reconstruction error using the redundancy and reconstructs a faithful Channel Frequency Response as measured at the user terminals. Then, the representation of this Channel Frequency Response is converted in the frequency domain to deduce a channel quality indicator for the different sub-channels constituting the multi-carrier system.  
         [0016]     The method according to the present invention presents the advantage to reduce the signaling load on the feedback signaling channel while having a very fine frequency resolution. This results in an efficient resource allocation leading to an increased throughput in the network.  
         [0017]     Further advantageous features of the invention are defined in the dependent claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     Other characteristics and advantages of the invention will appear on reading the following description of a preferred embodiment given by way of non-limiting illustrations, and from the accompanying drawings, in which:  
         [0019]      FIG. 1  shows an illustration of a network using multi-carrier transmission in the downlink;  
         [0020]      FIG. 2  shows an example representing the communication between a user terminal and a base station according to the present invention;  
         [0021]      FIG. 3  details the different steps of the method performed at user terminal  11  and at base station  12  according to the method of the present invention.  
         [0022]      FIG. 4  shows the error bound using the oversampling and the windowing operation. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0023]      FIG. 1  shows an illustration of a network using multi-carrier transmission in the downlink and a feedback signaling channel in the uplink. User terminals  11  are receiving multi-carrier frequency signals in the downlink from base station  12  over a plurality of sub-carriers  13 . On the uplink, user terminals  11  send signaling information to base station  12  on a feedback signaling channel  14 .  
         [0024]     The multi-carrier network may comprises 256, 512 or up to thousands of sub-carriers which are allocated to user terminals  11 . Preferably, several sub-carriers  13  are allocated to each user terminal  11  in order to match the high throughput received from base station  12  in the downlink. The frequency separation between the sub-carriers is chosen so that the sub-carriers are orthogonal to one another (i.e. the data transmitted on one sub-carrier is not causing interference on the data sent on the other sub-carriers).  
         [0025]     The feedback channel  14  may be advantageously implemented according to HSDPA (High Speed Downlink Packet Access) specification where the channel quality information according to the invention are sent back from the user terminals  11  to the base station  12  over a HS-DPCCH (High speed dedicated Physical Control Channel) having a capacity of around 1500 bit/s.  
         [0026]      FIG. 2  shows a flow diagram representing the communication between the user terminal  11  and the base station  12  as well as the steps performed according to the present invention.  
         [0027]     At step  21 , base station  12  informs user terminal  11  on parameters which should be used for performing the method according to the present invention. These parameters will be addressed in the following  
         [0028]     At step  22 , user terminal  11  performs steps  31  to  33  which will detailed in connection with  FIG. 3  in order to generate Channel Frequency Response information to be sent on the feedback signaling channel to base station  12 .  
         [0029]     At step  23 , user terminal sends the Channel Frequency Response information on the feedback channel.  
         [0030]     At step  24 , base station  12  performs steps  34 ,  341  to  343  and  35  which will detailed in connection with  FIG. 3  in order to gain reliable channel quality information for all sub-carriers of the multi-carrier system Steps  22  to  24  are repeated at regular time intervals in order to track the change of the channel quality.  
         [0031]     Then, the method starts from the beginning when base station  12  detects that new parameters would be better adopted to the new channel conditions.  
         [0032]      FIG. 3  details the different steps of the method performed at user terminal  11  and at base station  12 .  
         [0033]     According to the present invention, user terminals  11  are measuring channel frequency responses (CFR) on the entire frequency band of the system. These measurements are performed using prior art methods well known for the persons skilled in the art. These measurements are preferably performed at regular time interval. The Channel Frequency Response is measured in the frequency domain. Alternatively, a channel impuls response may be measured in the time domain and transformed in the frequency domain using a Fast Fourier Transformation in order to obtain a channel frequency response.  
         [0034]     Then, the channel frequency response is approximated by preferably a real trigonometric polynomial of degree L (step  31 ), where L is a predefined constant to provide a reliable accuracy after the approximation. Other approximation functions such as Wavelets are also possible. The 2L+1 coefficients of the real trigonometric polynomial are found using e.g. a least-squares based approximation (Alternative 1) or a linear programming based algorithm (Alternative 2).  
         [0035]     Alternative 1: The least-squares based algorithm minimizes the sum of quadratic errors at the frequency positions corresponding to each sub-carrier. The searched coefficients are obtained by the matrix multiplication: x*=Q·b where Q is a constant matrix defined as Q=(A T ·A) −1 A, where A is the matrix of trigonometric terms  
       A   =     (           1   2           cos   ⁢           ⁢     ω   0           ⋯         cos   ⁢           ⁢   L   ⁢           ⁢     ω   0             sin   ⁢           ⁢     ω   0           ⋯         sin   ⁢           ⁢   L   ⁢           ⁢     ω   0               ⋮                               ⋰                               ⋮             1   2           cos   ⁢           ⁢     ω     K   -   1             ⋯         cos   ⁢           ⁢   L   ⁢           ⁢     ω     K   -   1               sin   ⁢           ⁢     ω     K   -   1             ⋯         sin   ⁢           ⁢   L   ⁢           ⁢     ω     K   -   1               )         
 
 and b consists of the SIR measurements of the CHANNEL FREQUENCY RESPONSE at the sub-carrier positions. 
 
         [0036]     Alternative 2: The linear programming based algorithm minimizes the maximum error at the frequency positions corresponding to each sub-carrier. This minimization problem can be posed as a common linear programming problem and thus can be solved using many well-known algorithms.  
         [0037]     In step  32 , a predefined N where N must be greater or equal to the number of samples are extracted from the real trigonometric polynomial. The number N is the predefined oversampling number which must be equal or greater than the total number of the coefficients 2L+1 of the real trigonetric polynomial. This oversampling introduces redundancy which can be used at the receiver (i.e. base station  12 ), e.g. to minimize the error probability of the reconstructed channel frequency response at the receiver (base station  12 ). The number N is preferably determined at base station  12  and communicated to user terminal  11 .  
         [0038]     Preferably, the N samples are chosen equally spaced in the frequency domain corresponding to the channel frequency response frequency band. Nevertheless, base station  12  may be interested in another repartition of the n samples. In this case, base station  12  informs user terminal  11  on the position in the frequency domain where the samples should be located.  
         [0039]     Then, the N samples are quantized (step  33 ) and transmitted over the feedback signaling channel  14  to base station  12 . Preferably, the quantization granularity is also determined at base station  12  and communicated to user terminal  11 . For compression reasons the granularity of the quantization may correspond to a number of predefined modi.  
         [0040]     Following steps are performed at base station  12  to recover the channel frequency response and determine a channel quality indication for the different sub-carriers of the system:  
         [0041]     Step  34  consists in sub-steps  341  to  343 .  
         [0042]     Step  341  consists in performing an inverse Fast Fourier Transformation to obtain the feedback information in the time domain. This enables it to obtain the original Channel Impulse Response and remaining terms due to oversampling. The Channel Impulse Response are not error free due to the quantization performed at the transmitter.  
         [0043]     Step  342  consists in performing a windowing operation on the obtained channel frequency response. The windowing operation is designed to minimize the maximum error in the time domain (or another error measure) and consists preferably in a function corresponding to a constant value in a first time domain and to a linear slope in a second time domain. The error performance corresponding to the oversampling number N is shown in  FIG. 4 . The first time domain is preferably equal to the duration of the transmitted data if the channel were error free.  
         [0044]     Step  343  consists in performing once more a Fast Fourier Transformation in order to deduce the channel quality indicator for each sub-carrier. The channel quality indicator for each sub-carrier being read on the curve representing the Fast Fourier Transformation at the frequency corresponding to each sub-carrier of the multi carrier transmission system.  
         [0045]     The method according to the present invention is particularly advantageous in systems with a large number of sub-carriers. The original channel frequency response information is strongly compressed after the approximation at the cost of the minimized error. Instead of K—which is the original number of sub-carriers—samples only 2L+1 —where L is the degree of the real trigonometric polynomial—samples are necessary for the feedback.  
         [0046]     Furthermore the error due to quantization can be proven to be restricted to a maximum error bound thanks to the redundancy introduced by the oversampling and to the shape of the windowing function. Different windowing operation results in optimizing other error measures than the maximum error.  
         [0047]     The method according to the present invention may be used for different purposes. First of all, this method can help optimizing the allocation of sub-carriers to the different user terminals in a way which improve the capacity in the downlink. Only sub-carriers presenting a good channel quality towards a mobile station being allocated to this user terminal.  
         [0048]     Secondly, the method provides a precise description of the channel quality which allows the base station  12  to optimally allocate the transmission power (with e.g. the well-known waterfilling algorithm) in order to reduce the total transmission power and further the inter-cell interference.  
         [0049]     Thirdly, the method can be used in combination with other inventions to describe the channel quality as a means to reduce the effective number of channel coefficients in combination with the reconstruction process resulting in a tightly controlled maximum error.  
         [0050]     To summarize the method according to the present invention enables it to enhance the performance of OFDM downlink system by frequency selective resource allocation algorithm such as adaptive modulation, sub-carrier allocation or power control. According to the channel conditions reported from the user terminal to the base station, the resource can be optimally distributed to achieve maximal throughput.