Abstract:
The present application discloses methods and apparatus for modifying a normal link adaptation process of a wireless device ( 200 ). The normal link adaptation process may be used to compensate for a fast fading dip detected by a base station ( 100 ). The present application teaches that the normal fast fading compensation may cause inter-cell interference and degrade system performance. The present application discloses imposing a fast fading restriction or limitation on the normal fast fading compensation can reduce inter-cell interference, improve system capacity, and extends the battery life of the wireless device.

Description:
TECHNICAL FIELD 
       [0001]    The present application relates generally to enhancement of system performance in a wireless network and, more specifically, to reduction of inter-cell interference in a wireless network to optimize system capacity. 
       BACKGROUND 
       [0002]    Power control is used on the uplink link in a wireless communication system to control the power of signals received at each base station from the wireless devices. As a wireless device moves within the network, the channel conditions change continuously due to fast and slow fading, shadowing, number of users, external interference, and other factors. Closed loop power control algorithms dynamically control the transmit power of the wireless device on the uplink link. Closed loop power control includes inner loop and outer loop power control mechanism. For inner loop power control, the base station measures the SIR of the received signal, compares the measured SIR to a SIR target, and adjusts the transmit power of the wireless device depending on the comparison. Outer loop power control adjusts the SIR target for the inner loop power control mechanism to maintain desired performance criteria, such as a desired Frame Error Rate (FER). 
         [0003]    The inner loop power control mechanism provides improved performance for fast fading channels. Typically, in inner loop power control, the base station can send as many as 1500 up/down power control commands per second to the wireless device. The use of up/down power control commands keeps the received power level constant at the base station. When the received power level at the base station remains stable, the number of re-transmissions by the wireless device due to transmission errors can be maintained, e.g., below a threshold. Power control can also reduce intra-cell interference between uplink transmissions. 
         [0004]    One problem with power control is that when fast fading occurs, the power of the wireless device may be increased many decibels (dBs) to compensate the path loss due to fast fading. The increase may be as large as 30 dBs. Because the fast fading loss to different cells has low correlation, a large increase of transmit power by the wireless device to maintain the signal level at the serving base station may result in significant interference with neighboring cells. 
         [0005]    Therefore, the conventional method of increasing a wireless device&#39;s uplink transmit power to compensate for fast fading leads to strong inter-cell interference. The affected neighboring cells may need to combat the interference with additional resources. Improved methods and apparatus are needed for efficient utilization of resources and improved system capacity. 
       SUMMARY 
       [0006]    The present application discloses methods and apparatus for improving a normal link adaptation process. The normal link adaptation process may be used to compensate for fast fading dips. 
         [0007]    In some embodiments, a method for modifying a link adaptation process is disclosed. The method is implemented at a wireless device and modifies a link adaptation process that is used for uplink transmissions from the wireless device. During a normal link adaption process, the wireless device receives one or more transmission parameters from a base station. Based on the one or more received transmission parameters, the wireless device determines a slot transmit power that is used for transmitting a radio signal on a radio channel. The wireless device also calculates an average transmit power and compares the average transmit power to the slot transmit power. Based on the comparison, the wireless device derives a transmission parameter. The wireless device then transmits a data packet in accordance with the derived transmission parameter. 
         [0008]    In some embodiments, a wireless device configured to modify a link adaptation process is disclosed. The wireless device comprises a transceiver and a processing circuit. The transceiver is configured to receive and transmit signals. The processing circuit is configured to modify the link adaption process. The processing circuit is configured to determine a slot transmit power for transmitting a radio signal on a radio channel and calculate an average transmit power. The processing circuit is further configured to derive a transmission parameter based on a comparison of the slot transmit power and the average transmit power. 
         [0009]    In some embodiments, a method is implemented at a base station for controlling a modified link adaptation process of a wireless device. The base station determines one or more controlling parameters for controlling the modified link adaptation process at the wireless device. The one or more controlling parameters are then transmitted to the wireless device for use in modifying the link adaptation process. 
         [0010]    In some embodiments, a base station configured to control a modified link adaptation process of a wireless device is disclosed. The base station comprises a transceiver for transmitting data and control signals to the wireless device. The base station also comprises a processing circuit for determining one or more controlling parameters. The controlling parameters are transmitted to the wireless device and used by the wireless device to modify a link adaptation process. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  illustrates an exemplary wireless communication network implementing inner loop power control. 
           [0012]      FIG. 2  illustrates a link adaptation process in which a fast fading dip detected by a base station and a fast fading compensation implemented by a wireless device. 
           [0013]      FIG. 3  illustrates an exemplary method of modifying a link adaptation process at a wireless device. 
           [0014]      FIG. 4  illustrates an exemplary wireless device configured to modify a link adaptation process. 
           [0015]      FIG. 5  illustrates an exemplary method implemented at a base station for controlling a modified link adaptation process of a wireless device. 
           [0016]      FIG. 6  illustrates an exemplary base station configured to control a modified link adaptation process of a wireless device. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Referring now to the drawings, the present invention will be described in the context of a wireless communication network  10  implementing High Speed Packet Access (HSPA) services. The wireless communication network may, for example, operate according to the Wideband Code Division Multiple Access (VVCDMA) standard, Long Term Evolution (LTE) standard, or other standard providing HSPA services. The wireless communication network  10  comprises a plurality of base stations  100  providing service in respective cells  12  of the wireless communication network. The base stations  100  are sometimes referred to as NodeBs (NBs), Evolved NodeBs (eNBs), or access nodes. 
         [0018]      FIG. 1  illustrates two cells  12 , denoted as Cell A and Cell B, served by respective base stations  100 . A wireless device  200  is connected to the base station  100  in Cell A. The base station  100  in Cell A receives uplink transmissions from the wireless device  200  on an uplink channel, for example, a Dedicated Packet Control Channel (DPCCH) in WCDMA systems, and implements closed loop power control to maintain the signal level at the base station at a desired level. The signal level of the received signal may be measured as Signal to Interference Ratio (SIR) or Received Signal Code Power (RSCP). In one embodiment, the base station  100  measures the SIR of the received signal, compares the measured SIR to a SIR target, and adjusts the transmit power of the wireless device  200  depending on the comparison. When the SIR is above the SIR target, the base station  100  sends a down command and the wireless device  200  decreases its transmit power by one step. When the SIR is below the SIR target, the base station  100  sends an up command and the wireless device  200 , which increases its transmit power by one step. The base station  100  also implements outer loop power control to adjust the SIR target for the inner loop power control mechanism to maintain a desired performance criterion, such as a desired Frame Error Rate (FER). 
         [0019]    When the wireless device  200  experiences fast fading, the received signal power will deteriorate rapidly, and the base station  100  will increase the transmit power of the wireless device  200  to maintain the desired signal level at the base station  100 . Fast fading refers to the phenomenon in which the time scale of the variation of the radio condition is small compared to the time scale of the application utilizing the channel. 
         [0020]      FIG. 2  illustrates how inner loop power control compensates for path loss due to fast fading.  FIG. 2(   a ) illustrates the received signal strength at the base station  100 .  FIG. 2(   b ) illustrates the transmit power of the wireless device  200 . In  FIG. 2(   a ), a fast fading dip takes place at time t 0 . In some embodiments, the dip in the received signal power may be detected using SIR measurements or RSCP measurements. As the received signal power drops, the measured SIR also drops and the base station  100  sends power up commands to the wireless device  200  to compensate for the path loss due to fast fading. Upon receipt of the power up commands received from the base station  100 , the wireless device  200  increases its transmit power to counteract the fast fading.  FIG. 2(   b ) illustrates how the transmit power of the wireless device  200  changes with time. The increase of transmit power occurs at a time slightly later than t 0 . The increased transmit power compensates the path loss due to fast fading. The signal power received at the base station  100  returns to the pre fast-fading-dip level as shown in  FIG. 2(   b ). 
         [0021]    When the transmit power of the wireless device  200  is increased, the interference on the neighboring cell increases as well. Thus, during fast fading events, the uplink transmissions from the wireless device  200  may interfere with the radio communication in neighboring cells. The transmit power of the wireless device  200  may be increased as much as 30 dB during the fading event, which would generate strong interference in the neighboring cell. In exemplary embodiments of the present disclosure, the wireless device  200  may be configured to limit the transmit power during fast fading event to mitigate interference caused by the wireless device  200 . In some embodiments, the wireless device  200  calculates a slot transmit power for an uplink transmission based on the directions or commands from the base station  100 . The wireless device  200  compares the calculated slot transmit power to an average transmit power of the wireless device  200  over a predetermined period. Based on the comparison, the wireless device  200  adjusts a transmission parameter to reduce the required slot transmit power to avoid creating excessive interference. Under normal conditions, the wireless device  200  uses the calculated slot transmit power for its uplink transmission. During a fast fading event, the wireless device  200  uses an adjusted slot transmit power. 
         [0022]      FIG. 3  illustrates an exemplary method implemented at the wireless device  200  for modifying a normal link adaptation process to limit interference during fast fading events. The modified link adaptation process may be used to limit a fast fading compensation. In  FIG. 3 , the wireless device  200  calculates a slot transmit power for transmitting a radio signal on a radio channel (Step  310 ), and calculates an average transmit power (Step  320 ). The wireless device  200  then derives a transmission parameter based on a comparison of the slot transmit power and the average transmit power (Step  330 ). The transmission parameter is used by the wireless device  200  to transmit a data packet (Step  340 ). 
         [0023]    To determine an average transmit power, the wireless device  200  filters its transmit power on an uplink Dedicated Packet Control Channel (DPCCH). In one embodiment, the wireless device  200  calculates a filtered transmit power as follows: 
         [0000]        P   TX     —     filter ( n )α* P   TX     —     filter ( n− 1)+(1−α)* P   TX     —     measured ( n )  Eq (1)
 
         [0000]    where P TX     —     filter (n) represents the filtered DPCCH transmit power at time interval n. P TX     —     filter (n−1) represents the filtered DPCCH transmit power at time interval n−1. P TX     —     measured (n) represents the measured DPCCH transmit power at time interval n. The weighting factor α determines the length of the filter. The larger the weighting factor α is, the longer the length of the filter becomes. The weighting factor α may be determined by the base station  100  and signaled to the wireless device  200  over a control channel. The weighting factor α may be broadcast to the wireless device  200 . Alternatively, the weighting factor α may be determined and signaled by a radio network controller (RNC) via Radio Resource Control (RRC) signaling. Alternatively, the weighting factor α may be hard-coded in the wireless device  200 . The filtered transmit power at time interval n, P TX     —     filter (n), represents an average transmit power. 
         [0024]    The wireless device  200  compares the filtered transmit power at time interval n with the calculated slot transmit as determined by inner loop power control to detect an increase in the slot transmit power. For example, the wireless device  200  may calculate a transmit power ratio R transmit     —     power  according to: 
         [0000]    
       
         
           
             
               
                 
                   
                     R 
                     
                       transmit 
                        
                       _ 
                        
                       powe 
                        
                       r 
                     
                   
                   = 
                   
                     
                       P 
                       
                         
                           slot 
                            
                           _ 
                            
                           transmit 
                         
                          
                         
                           _ 
                            
                           power 
                         
                       
                     
                     
                       
                         P 
                         
                           TX 
                            
                           _ 
                            
                           filte 
                            
                           r 
                         
                       
                        
                       
                         ( 
                         n 
                         ) 
                       
                     
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                    
                   
                     ( 
                     2 
                     ) 
                   
                 
               
             
           
         
       
     
         [0000]    The transmit power ratio R transmit     —     power  given by Eq. (2) reflects how much the slot transmit power deviates from the average transmit power. The wireless device  200  compares the transmit power ratio R transmit     —     power  to a threshold. Based on the comparison, the wireless device  200  determines whether to modify a normal link adaptation process and apply a restriction on the fast fading compensation. For example, the wireless device  200  may compute a new transmission parameter based on this ratio. 
         [0025]    There are several different approaches that can be used to limit the interference that would otherwise occur in response to a fast fading dip. In one approach, the wireless device  200  modifies a normal link adaptation process by reducing or limiting the data rate/transport format determined by the normal link adaptation process. The data rate/transport format may be a data rate/transport format on a data channel, e.g., an Enhanced Data Packet Data Channel (E-DPDCH). Limiting the data rate means that fewer bits will be transmitted reducing the total interference towards neighboring cells. In another approach, the wireless device  200  reduces or limits the total energy or power used for data transmission on an E-DPDCH. Both approaches are explained in detail below. 
         [0026]    In some embodiments, the wireless device  200  adjusts the data rate for the uplink transmission in order to limit the transmit power increase that would have occurred during a normal link adaptation process. To adjust the data rate, the wireless device  100  calculates a rate correction factor based on the transmit power ratio R transmit     —     power  and uses the rate correction factor CF to calculate the data rate/transport format for the uplink transmission. The rate correction factor CF may be calculated according to: 
         [0000]        CF =max(1; k*R   transmit     —     power ).  Eq. (3)
 
         [0000]    The data rate R may then be calculated according to: 
         [0000]    
       
         
           
             
               
                 
                   
                     R 
                     = 
                     
                       max 
                        
                       
                         ( 
                         
                           
                             
                               R 
                               normal 
                             
                             
                               C 
                                
                               
                                   
                               
                                
                               F 
                             
                           
                           ; 
                           min_rate 
                         
                         ) 
                       
                     
                   
                   , 
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                    
                   
                     ( 
                     4 
                     ) 
                   
                 
               
             
           
         
       
     
         [0000]    where R normal  is the normal data rate that would have been selected without compensation for fast fading, min_rate is the lowest data rate that is allowable, and k is a constant used to scale the ratio of the slot transmit power to the average transmit power. The constant k controls the extent to which the data rate in the transport format should be restricted. It is noted that in general higher data rate means higher transmit power. The constant k limits the fast fading compensation. The data rate R may be used to select the transport format. Both the min_rate and the constant k may be provided to the wireless device  200  by the base station  100  over a control channel. The two parameters can be broadcast to the wireless device  100 . Alternatively, a radio network controller may determine the min_rate and constant k and provide them to the wireless device  100  via RRC signaling. In some embodiments, these two parameters can also be hardcoded in the wireless device  200  as well. 
         [0027]    In some embodiments, the rate correction factor CF may be provided to the transport format selection function for the wireless device  200 . In this case, the selection function uses the rate correction factor CF as a scaling factor to scale the available power headroom. The scaled available power headroom is then used to perform a data rate/transport format selection. In one exemplary scenario, when there is an increase in the ratio of the slot transmit power to the average transmit power, the scaling factor increases the available power headroom. More reserved power headroom restricts the maximum transmit power available for the wireless device  200 . This in turn would limit the data rate in the selected transport format. 
         [0028]    In other embodiments, rather than adjusting the data rate/transport format, the wireless device  200  uses the transmit power ratio R transmit     —     power  to directly adjust the transmit power on the Enhanced Data Packet Data Channel (E-DPCCH). To adjust the transmit power for the E-DPCCH, the wireless device  100  calculates a power correction factor based on the transmit power ratio R transmit     —     power . The power correction factor PCF may be calculated according to: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       P 
                        
                       
                           
                       
                        
                       C 
                        
                       
                           
                       
                        
                       F 
                     
                     = 
                     
                       max 
                        
                       
                         ( 
                         
                           
                             min 
                              
                             
                               ( 
                               
                                 1 
                                 ; 
                                 
                                   y 
                                   * 
                                   
                                     1 
                                     
                                       R 
                                       
                                         transmit 
                                          
                                         _ 
                                          
                                         power 
                                       
                                     
                                   
                                 
                               
                               ) 
                             
                           
                           ; 
                           min_PCF 
                         
                         ) 
                       
                     
                   
                   , 
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                    
                   
                     ( 
                     5 
                     ) 
                   
                 
               
             
           
         
       
     
         [0000]    where y is a constant that controls how much the E-DPCCH transmit power should be restricted, and min_PCF represents the smallest value the power correction factor PCF can take. min_PCF also reflects the largest power reduction that the wireless device  200  is allowed to make when restricting the change in E-DPCCH transmit power. The modified E-DPCCH transmit power may then be calculated according to: 
         [0000]        P   E-DPCCH   =PCF*P   normal     —     E-DPCCH ,  Eq. (6)
 
         [0000]    where P normal     —     E-DPCCH  represents the power level that should be selected without any fast fading power limitation. 
         [0029]      FIG. 4  illustrates an exemplary wireless device  200  configured to modify a link adaptation process as herein described. The wireless device  200  comprises a transceiver circuit  210  and a processing circuit  220 . The transceiver circuit  210  is configured to receive and transmit signals to and from a base station, e.g., the base station  100 . The processing circuit  220  is configured to modify a link adaptation process. The processing circuit  220  may comprise a link adaptation circuit  230  and a correction circuit  240 . The link adaptation circuit  230  is configured to select a normal transport format for the wireless device&#39;s uplink transmissions. In some scenarios, the normal transport format reflects a fast fading compensation. The correction circuit  240  is configured to modify the transport format selected by the link adaptation circuit  230 . The correction circuit  240  is configured to determine a slot transmit power for transmitting a radio signal on a radio channel. The correction circuit  240  calculates an average transmit power and derives a transmission parameter based on a comparison of the slot transmit power and the average transmit power. The derived transmission parameter is used by the wireless device  200  to transmit a data packet. 
         [0030]    In some embodiments, the base station  100  may set conditions on when the wireless device  200  can restrict a fast fading compensation. For example, the base station  100  may decide that the wireless device  200  only applies a fast fading restriction when the measured path loss of an uplink transmission is larger than a threshold. Alternatively, the wireless device  200  may apply a fast fading restriction only when the wireless device  200  is transmitting above a pre-determined minimum power level. The path loss threshold and the pre-determined minimum power level may be transmitted to the wireless device  200  by the base station  100 . These two parameters may be broadcast to the wireless device  200 . Alternatively, a radio network controller may determine and transmit the path loss threshold and the pre-determined minimum power level to the wireless device  200  via RRC signaling. The path loss threshold and the pre-determined power level may be hard-coded in the wireless device  200  as well. As described above, the modified transmit power may represent a fast fading restriction imposed by the wireless device  200  to reduce or limit the increase of transmit power for fast fading compensation. In a fast fading restriction, one or more controlling parameters determine how large and/or when a fast fading restriction should be applied. For example, the path loss threshold, the pre-determined minimum power level, min_rate, constant k, min_PCF and constanty are all controlling parameters the wireless device  200  relies on to modify a normal link adaptation process. Those controlling parameters are determined and transmitted to the wireless device  200  by a radio network, either from a RNC node (via a base station) or directly from a base station, e.g., the base station  100 . An exemplary base station  100  configured to control a modified link adaptation process of the wireless device  200  is illustrated in  FIG. 5 . 
         [0031]    The base station  100  comprises a transceiver circuit  110  and a processing circuit  120 . The transceiver circuit  110  is configured to transmit data and control signals to the wireless device  200 . The processing circuit  120  is configured to determine one or more controlling parameters and transmit the one or more controlling parameters for the wireless device  200  to use in modifying a link adaptation process. The one or more controlling parameters may be transmitted to the wireless device  200  via RRC signaling or broadcasting. 
         [0032]      FIG. 6  illustrates an exemplary method  400  implemented at the base station  100  for controlling a modified link adaptation process at the wireless device  200 . The base station  100  determines one or more controlling parameters for the wireless device (Step  410 ). The base station  100  then transmits the one or more controlling parameters to the wireless device to control a modified link adaptation process at the wireless device  200  (Step  420 ). Examples of the one or more controlling parameters include the path loss threshold, the pre-determined minimum power level, min_rate, constant k, min_PCF, and constant y, all described in detail in the above discussion. 
         [0033]    It is noted that some or all of the above mentioned controlling parameters may be hardcoded in the wireless device  200  as well. 
         [0034]    It is also noted that a radio network controller may be configured to determine the one or more controlling parameters for the wireless device  200  and transmit the one or more controlling parameters to the wireless device  200  via RRC signaling. The radio network controller may comprise a network interface for communicating with the base station  100  and a processing circuit for determining the one or more controlling parameters. The one or more controlling parameters are sent to the base station  100  via the network interface for transmitting to the wireless device  200 . 
         [0035]    It is further noted that in some embodiments, other types of network nodes, such as eNBs, NodeBs, access nodes, etc., may be configured to control the wireless device  200  to modify a link adaptation process at the wireless device  200 . 
         [0036]    When a fast fading restriction is applied, the modified link adaptation process does not fully compensate for fast fading dips. This may result in a lower data transmission rate. Or it may lead to increased re-transmission attempts. The effect of both consequences is that some data may be transmitted at a different time using different resources, for example, when the fast fading has subsided. The wireless device  200  avoids inefficient resource-utilization during fast-fading and arranges data transmission at another time when the channel conditions have improved. Another advantage of fast fading restriction is reduced interference level as experienced by neighboring cells during fast fading in cell A, which would lead to an improved overall system performance, especially in a multi-user multi-cell scenario. Fast fading restriction also lowers or limits the transmit power of the wireless device  200 , allowing the wireless device  200  to conserve battery power and extend the battery life. 
         [0037]    The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.