Patent Publication Number: US-2009225716-A1

Title: Base station link adaptation method

Description:
The invention is based on a priority application EP 08 290 214.9 which is hereby incorporated by reference. 
     TECHNICAL FIELD 
     The invention relates to a method of link adaptation in a base station and to a computer program product. 
     BACKGROUND OF THE INVENTION 
     Link adaptation, or adaptive modulation and coding (AMC), denotes the selection of the modulation, coding and other signal and protocol parameters according to the conditions on the radio link. The conditions may be related to the interference due signals coming from other transmitters, the sensitivity of the receiver, or the available transmitter power margin. Wireless communication standards as e.g. WiMax and LTE use a rate adaptation algorithm that adapts the modulation and coding scheme (MCS) according to the quality of the radio channel, and thus the bit rate and robustness of data transmission. The process of link adaptation is dynamic and the signal and protocol parameters change as the radio link conditions. 
     Adaptive modulation systems require channel information at the transmitter. This could be acquired, for example, by assuming that the channel from the transmitter to the receiver is approximately the same as the channel from the receiver to the transmitter. Alternatively, the channel information can also be directly measured at the receiver, and sent back to the transmitter. Adaptive modulation systems improve rate of transmission, and bit error rates, by exploiting the channel information that is present at the transmitter. Especially over fading channels which model wireless propagation environments, adaptive modulation systems shows great performance enhancements compared to systems that do not exploit channel knowledge at the transmitter. 
     There is therefore a need for an improved method of selecting the link adaptation characteristics in a base station, to a base station and to a computer program product to perform the method in accordance with the invention. 
     SUMMARY OF THE INVENTION 
     The invention relates to a method of selecting link adaptation characteristics in a base station of an OFDM mobile digital communication, the base station being coupled to at least one mobile station, the method comprising the steps of: measuring a mobile station&#39;s velocity in the mobile station; receiving a mobile station&#39;s velocity value from the mobile station by the base station; calculating a channel estimation and equalization with a first algorithm by the base station, if the mobile station&#39;s velocity value is smaller than a first threshold; activating a MIMO Matrix B including spatial multiplexing and activating an AMC permutation mode by the base station, if the mobile station&#39;s velocity value is smaller than the first threshold, and if the mobile station occupies multiple antennas; activating a beamforming algorithm and activating the AMC permutation mode by the base station, if the mobile station&#39;s velocity value is smaller than the first threshold, and if the mobile station does not occupy multiple antennas. 
     The method further comprises the steps of: activating a RX/TX diversity algorithm and activating a PUSC permutation mode by the base station, if the mobile station&#39;s velocity value is bigger than the first threshold, and if the mobile station does not occupy multiple antennas; activating the MIMO Matrix B including spatial multiplexing and activating the PUSC permutation mode by the base station, if the mobile station&#39;s velocity value is bigger than the first threshold, if the mobile station occupies multiple antennas, and if the mobile station&#39;s velocity value is smaller than a second threshold; activating a MIMO Matrix A including space time block coding and activating the PUSC permutation mode by the base station, if the mobile station&#39;s velocity value is bigger than the first threshold, if the mobile station occupies multiple antennas, and if the mobile station&#39;s velocity value is bigger than the second threshold. 
     The main advantage of the embodiments is that the velocity of the mobile station&#39;s is measured by the base station, the value of the measurement is sent to the base station and therefore available immediate for further processing. This may include estimating the right channel estimation and equalization algorithms, the permutation mode to use, or the suitable MIMO matrix. In contrast to all other measurements, as e.g. the CINR variance, require several frame lengths in calculations before they are available as an input. 
     In accordance with an embodiment, the method further comprises the steps of: calculating the channel estimation and equalization using a second algorithm by the base station, if the mobile station&#39;s velocity value is bigger than the first threshold and if the mobile station&#39;s velocity value is smaller than the second threshold, wherein the second algorithm provides more accurate channel estimation for this velocity range than the first algorithm, wherein the second threshold is bigger than the first threshold; calculating the channel estimation and equalization using a third algorithm by the base station, if the mobile station&#39;s velocity value is bigger than the second threshold, and if the mobile station&#39;s velocity value is smaller than a third threshold, wherein the third algorithm provides more accurate channel estimation for this velocity range than the second algorithm, wherein the third threshold is bigger than the second threshold; and calculating the channel estimation and equalization using a fourth algorithm by the base station, if the mobile station&#39;s velocity value is bigger than the third threshold, wherein the fourth algorithm provides more accurate channel estimation for this velocity range than the third algorithm. The main advantage of the embodiments is that different algorithms are proposed according to the velocity range of the mobile station, each one of the algorithms providing the most accurate channel estimation for that velocity range. 
     In accordance with further embodiments, the mobile station&#39;s velocity value is sent through an extended fast feedback channel, as e.g. the Channel Quality Indicator Channel (CQICH), to the base station. Alternatively, the mobile station&#39;s velocity value is sent with a MAC management message to the base station. 
     The method as in any of the preceding embodiments, wherein the mobile station&#39;s velocity is measured with a Global Positioning System (e.g. GPS, Galileo, Glonass) or motion detection sensors located in the mobile station. 
     In another aspect, the invention relates to a base station in an OFDM mobile digital communication, the base station coupled to at least one mobile station, the base station comprising: means for receiving a mobile station&#39;s velocity value measured by the mobile station; means for calculating a channel estimation and equalization with a first algorithm, if the mobile station&#39;s velocity value is smaller than a first threshold; means for activating a MIMO Matrix B including spatial multiplexing and activating an AMC permutation mode, if the mobile station&#39;s velocity value is smaller than the first threshold, and if the mobile station occupies multiple antennas; and means for activating a beamforming algorithm and activating the AMC permutation mode, if the mobile station&#39;s velocity value is smaller than the first threshold, and if the mobile station does not occupy multiple antennas. 
     The base station further comprises means for activating a RX/TX diversity algorithm and activating a PUSC permutation mode, if the mobile station&#39;s velocity value is bigger than the first threshold, and if the mobile station does not occupy multiple antennas; means for activating the MIMO Matrix B including spatial multiplexing and activating the PUSC permutation mode, if the mobile station&#39;s velocity value is bigger than the first threshold, if the mobile station occupies multiple antennas, and if the mobile station&#39;s velocity value is smaller than a second threshold; means for activating a MIMO Matrix A including space time block coding and activating the PUSC permutation mode, if the mobile station&#39;s velocity value is bigger than the first threshold, if the mobile station occupies multiple antennas, and if the mobile station&#39;s velocity value is bigger than the second threshold. 
     The base station of the embodiments further comprises: means for calculating the channel estimation and equalization using a second algorithm, if the mobile station&#39;s velocity value is bigger than the first threshold and if the mobile station&#39;s velocity value is smaller than the second threshold, wherein the second algorithm provides more accurate channel estimation for this velocity range than the first algorithm, wherein the second threshold is bigger than the first threshold. 
     The base station of the embodiments, wherein the mobile station&#39;s velocity value is sent with an extended fast feedback channel. 
     In another aspect, the invention relates to a computer program product stored on a computer usable medium, comprising computer readable program means for causing a computer to perform a method according to any of the embodiments when the program is run on the computer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following preferred embodiments of the invention will be described in greater detail by way of example only making reference to the drawings in which: 
         FIG. 1  shows an example of a method of link adaptation in a base station, 
         FIG. 2  shows a second flow diagram of a channel estimation calculation, 
         FIG. 3  shows a third flowchart of the selection of the burst profile selection, 
         FIG. 4  shows a fourth flowchart of the selection of the burst profile selection, 
         FIG. 5  depicts a wireless communication system according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows an example of the link adaptation process in a base station comprising a first step  101  where measurements are carried out including the carrier to interference plus noise ratio (CINR), the received signal strength indication (RSSI). Parallel to the first step  101 , a second step  102  receives the mobile station&#39;s velocity information. The mobile station velocity information may be measured using a Global Positioning System (e.g. GPS, Galileo or Glonass), or motion detection sensors located in the mobile station. In the third step  103 , the CINR variance is compared with the variant threshold E 1 . If the CINR variance is not larger than the variance threshold, it indicates  104  that the wireless channel is flat and may be for example located in a rural environment. 
     After the step  103 , in a further step  105 , the mobile station velocity is compared with a first velocity threshold V 1 , and if the velocity is smaller than the velocity threshold, then a normal communication is carrier out between the mobile and the base station. On the other hand, if the variance or the velocity are bigger that its respective thresholds, the algorithm uses a specific channel estimation calculation using the mobile station&#39;s velocity and specific burst profile characteristics. This calculation is described in the subsequent figures. 
       FIG. 2  shows a flow diagram  200  of a calculation for the channel estimation and equalization according to different ranges of velocity thresholds. This calculation may be a subsequent development of the calculated CINR variance as described in  FIG. 1 . On the first step  201 , the mobile station&#39;s velocity is compared with a first threshold V 1 . If the velocity is smaller than the first threshold, the step  202  uses a complicated channel estimation and equalization algorithm M 1 . On the other hand, if the mobile station&#39;s velocity is larger than the first threshold, the step  203  compares this mobile station&#39;s velocity with a second threshold V 2 . This second threshold is bigger than the first threshold. If the mobile station&#39;s velocity is smaller than the threshold then it indicates that the number of the valid pilots for the estimation decreases and a second algorithm M 2  is applied in the step  204 . This second algorithm M 2  provides more accurate channel estimation for this velocity range than the first algorithm M 1 . 
     If the mobile station&#39;s velocity is larger than the second threshold, the step  205  compares the mobile station&#39;s velocity with a third threshold V 3 . If the mobile station&#39;s velocity is smaller than the third threshold, then it indicates that the number of the valid pilots for estimation is even smaller and a third channel estimation and equalization algorithm M 3  is applied. This third algorithm M 3  provides more accurate channel estimation for this velocity range than the second algorithm M 2 . If the mobile station&#39;s velocity is larger than the third threshold then, in the step  207 , a fourth algorithm M 4  is applied and the number of valid pilots for estimation is even smaller than the rest of the steps of the calculation. This fourth algorithm M 4  uses fewer resources than M 3  and it is ideal for the fastest varying channels. 
       FIG. 3  shows a third flow diagram for the method after selecting the burst profile in AMC permutation mode. On the first step  301  the mobile station&#39;s mobility is compared with a first threshold V 4 . If the mobile station&#39;s velocity is smaller than the first threshold, then step  302  applies an AMC permutation mode with frequency selective scheduling. In this case, the third step  303 , the modulation and coding scheme (MCS) can be determined by considering the velocity, CINR mean values and the RSSI level. The modulation scheme can be represented as a mapping between the coded information bits and the complex symbols. For example, the 2, 4 and 6 (or more) coded information bits are mapped to a complex symbol in QPSK, 16QAM and 64QAM (or higher) scheme, respectively. The symbol rate will be higher than the input bit rate, thus modulation directly increase the data rate. 
     The coding introduces redundancy to protect the original information bits, combating fading, noise and interference. The more redundancy, the better are the information bits protected. Due to the coding, the bit rate after coding is always lower. Modulation and coding are trade-off schemes. They depend on multi-dimensional aspects, such as fading, noise, interference and the signal power. Some of these aspects, as e.g. fading, are strongly influenced by the MS&#39;s velocity. It is then possible to use the velocity information as a reference to adaptively choose the best modulation and coding schemes. 
     On a fourth step  304  of  FIG. 3 , an indication is requested  304  in order to know if the active mobile station occupies multiple antennas. If the active mobile station occupies only single antenna, then the beamforming technique is available. Otherwise, the MIMO technique is available. The condition in the embodiments can thus determine, whether the mobile station supports MIMO or not. In the case that the active mobile stations do not occupy multiple antennas, a sixth step  306  uses a beamforming algorithm in order to suppress the interference that may occur from the MMSE that is benefited from the channel estimation. Beamforming is possible if the distance between the antenna elements is half-wavelength of the carrier frequency, so that the received signal of the BS antenna elements can be regarded as correlated signals. The base station gets the user location via Direction of Arrival (DoA) estimation, and places a beam to coherently collect the signal components. MMSE is an abbreviation of Minimum Mean Square Error. MMSE is an algorithm for receivers with multiple antennas that adaptively minimizes the mean square value of the error. 
     In a continuation of the burst profile selection of  FIG. 3 , if the indication of the active mobile station occupying multiple antennas is affirmative, a multiple input/multiple output (MIMO) matrix B with a spatial multiplexing can be activated in order to achieve high throughput over the bandwidth. 
       FIG. 4  shows a fourth flowchart of the selection of the burst profile in PUSC permutations. In a first step  401 , the mobile station&#39;s velocity is compared with a first threshold V 4 . If the mobile station velocity is larger than the threshold then a PUSC permutation mode in step  402  is used. On the third step  403 , the modulation and coding scheme (MCS) can be determined by using the measurements of the velocity, CINR mean values and the RSSI levels. On the fourth step  404  it is required to obtain information of the active mobile station using multiple antennas. If the information is affirmative, the fifth step  405  compares the mobile station&#39;s velocity with a second threshold V 5 . This second threshold is bigger than the first threshold. 
     If the mobile station&#39;s velocity is larger than the second threshold, then a sixth step  406  activates a multiple input and multiple output (MIMO) matrix A. The matrix A comprises space time block coding in order to introduce additional high spatial diversity. In the wireless communications as e.g. the WiMAX standard, matrix A is equivalent with Space-Time Block Coding (STBC). It is a one-stream MIMO scheme. The original signal is encoded according to the code matrix (Matrix A), and transmitted via multiple antennas. At the receiver, the spatial redundancy can improve the detection and recover the signal. 
     If the mobile station&#39;s velocity is smaller than the second threshold, then a seventh step  407  uses a MIMO matrix B with spatial multiplexing in order to achieve high throughput over the bandwidth. Matrix B is equivalent with Spatial Multiplexing (SM). It is a multi-stream MIMO scheme. The streams are encoded according to the code matrix (Matrix B), and transmitted via multiple antennas. This scheme has high data rate. The Matrix A and the Matrix B depends on criteria to perform the MIMO switch and in the embodiments, the mobile station&#39;s velocity is a key factor. Finally, if the mobile station does not occupy multiple antennas, then an eighth step  408  makes available a reception and transmission diversity algorithm. 
       FIG. 5  depicts a wireless communication system  500  comprising a base station  501  coupled to a mobile station  502 . The mobile station comprising means for  509  measuring the mobile station&#39;s velocity and means for  510  transmitting the mobile station&#39;s velocity value  511  to the base station  501 . The base station  501  comprises: means for receiving  503  a mobile station&#39;s velocity value measured by the mobile station; means for calculating  504  a channel estimation and equalization with a first algorithm, if the mobile station&#39;s velocity value is smaller than a first threshold; —means for activating  505  a MIMO Matrix B including spatial multiplexing and activating an AMC permutation mode, if the mobile station&#39;s velocity value is smaller than the first threshold, and if the mobile station occupies multiple antennas; and means for activating  506  a beamforming algorithm and activating the AMC permutation mode, if the mobile station&#39;s velocity value is smaller than the first threshold, and if the mobile station does not occupy multiple antennas. 
     The base station  501  further comprises means for activating  507  a RX/TX diversity algorithm and activating a PUSC permutation mode, if the mobile station&#39;s velocity value is bigger than the first threshold, and if the mobile station does not occupy multiple antennas; means for activating  508  the MIMO Matrix B including spatial multiplexing and activating the PUSC permutation mode, if the mobile station&#39;s velocity value is bigger than the first threshold, if the mobile station occupies multiple antennas, and if the mobile station&#39;s velocity value is smaller than a second threshold; and means for activating  509  a MIMO Matrix A including space time block coding and activating the PUSC permutation mode, if the mobile station&#39;s velocity value is bigger than the first threshold, if the mobile station uses a, if the mobile station occupies multiple antennas, and if the mobile station&#39;s velocity value is bigger than the second threshold. 
     LIST OF REFERENCE NUMERALS 
       
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 101 
                 First step 
               
               
                   
                 102 
                 Second step 
               
               
                   
                 103 
                 Comparison step 
               
               
                   
                 104 
                 Indication step 
               
               
                   
                 105 
                 Comparison step 
               
               
                   
                 106 
                 Standards step 
               
               
                   
                 201 
                 Comparison step 
               
               
                   
                 202 
                 First calculation step 
               
               
                   
                 203 
                 Comparison step 
               
               
                   
                 204 
                 Second calculation step 
               
               
                   
                 205 
                 Comparison step 
               
               
                   
                 206 
                 Third calculation step 
               
               
                   
                 207 
                 Fourth calculation step 
               
               
                   
                 301 
                 Comparison step 
               
               
                   
                 302 
                 Permutation mode step 
               
               
                   
                 303 
                 MCS step 
               
               
                   
                 304 
                 Comparison step 
               
               
                   
                 305 
                 MIMO Matrix B step 
               
               
                   
                 306 
                 Beamforming step 
               
               
                   
                 401 
                 Comparison step 
               
               
                   
                 402 
                 PUSC step 
               
               
                   
                 403 
                 MCS step 
               
               
                   
                 404 
                 Multiple antennas step 
               
               
                   
                 405 
                 Comparison step 
               
               
                   
                 406 
                 MIMO Matrix A step 
               
               
                   
                 407 
                 MIMO Matrix B step 
               
               
                   
                 501 
                 Base station 
               
               
                   
                 502 
                 Mobile station 
               
               
                   
                 503 
                 Means for receiving 
               
               
                   
                 504 
                 Means for calculating 
               
               
                   
                 505 
                 Means for activating 
               
               
                   
                 506 
                 Means for activating 
               
               
                   
                 507 
                 Means for activating 
               
               
                   
                 508 
                 Means for activating 
               
               
                   
                 509 
                 Means for activating 
               
               
                   
                 511 
                 Mobile station&#39;s velocity value