Abstract:
The transmit power of a CDMA downlink channel is controlled from a base station by receiving a command signal from a mobile station requesting it to decrease the transmit power of the downlink channel. In response, the base station decreases the transmit power if a hypothetically decremented value of the transmit power is higher than a nominal lower limit of its power control range, and further decreases the transmit power if the downlink channel still has a quality higher than a specified value at the mobile station even when the hypothetically decremented value is lower than the nominal lower limit. The base station sets the transmit power equal to the nominal lower limit if the hypothetically decremented value is lower than the nominal lower limit and the downlink channel still has a quality higher than the specified value at the mobile station.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to CDMA (code division multiple access) communication systems, and more specifically to a downlink power control method and a system using the same. 
   2. Description of the Related Art 
   A transmit power control scheme for downlink (base-to-mobile) channels of CDMA communication systems is described in “3GPP RAN (3rd Generation Partnership Project Radio Access Network) 25.214 v1.3.1”. According to this document, each mobile station constantly monitors its downlink channel and determines its signal-to-interference ratio (SIR). The mobile station compares the SIR value with a prescribed target value and transmits a TPC (transmit power control) command signal through an uplink channel, requesting the base station to increase or decrease the power level of the downlink channel. The power level of a downlink channel is varied by a predetermined incremental unit for each TPC command signal. Power control will be repeatedly performed if the base station repeatedly receives TPC command signals until the upper or lower limit of a power control range is reached. The minimum power control limit is determined in consideration of the fact that, when a power decrease takes place in a downlink channel of excellent signal quality, the signal quality at the reduced level still allows the base station to respond to a possible degradation which may subsequently occur due to a sudden movement of the mobile station. The maximum power control limit of the base station is determined by taking account of interference between mobile stations which would be caused by possible racing conditions in which they compete for power increase. The number of channels allocated to the base station is also a determining factor of the maximum limit of the control range. 
   However, one shortcoming of the prior art scheme is that, since power control is effected in a specified range that prevents the base station to transmit its power at a level below the minimum power control limit, those mobile stations that are located near the base station would receive power more than what they actually need for their downlink channels. As a result, useful energy resource of a base station is wasted. Another shortcoming of the prior art is that, due to the presence of the upper limit, those mobile stations that are located far off the base station would receive power less than what they actually need for their downlink channels even when the transmit power level of the base station still has a sufficient amount of allowance with respect to its maximum power control limit. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to provide a transmit power control technique for a CDMA base station to achieve full and efficient utilization of its power resource. 
   According to a first aspect, the present invention provides a method of controlling the transmit power of a plurality of CDMA downlink channels from a base station within a control range between a nominal lower limit and a nominal upper limit, comprising the steps of receiving, at the base station, a command signal from a mobile station requesting the base station to decrease the transmit power of a downlink channel, and decreasing, at the base station, the transmit power of the downlink channel if the downlink channel has a quality higher than a specified threshold value at the mobile station. 
   According to a second aspect, the present invention provides a method of controlling the transmit power of a plurality of CDMA downlink channels from a base station within a control range between a nominal lower limit and a nominal upper limit, comprising the steps of receiving, at the base station, a command signal from the mobile station requesting the base station to increase the transmit power of the downlink channel, and increasing the transmit power if total transmit power of the downlink channels is lower than a specified threshold value. 
   According to a third, specific aspect, the present invention provides a method of controlling the transmit power of a plurality of CDMA downlink channels from a base station within a control range between a nominal lower limit and a nominal upper limit, comprising the steps of (a) receiving, at the base station, a command signal from a mobile station requesting the base station to decrease the transmit power of a downlink channel, (b) decreasing the transmit power of the downlink channel if a hypothetically decremented value of the transmit power is higher than the nominal lower limit, (c) decreasing the transmit power of the downlink channel if the quality of the downlink channel at the mobile station is lower than a specified threshold value even when the hypothetically decremented value is lower than the nominal lower limit; and (d) setting the transmit power of the downlink channel equal to the nominal lower limit if the hypothetically decremented value is lower than the nominal lower limit and the quality of the downlink channel at the mobile station is lower than the specified threshold value, receiving, at the base station, a command signal from the mobile station requesting the base station to increase the transmit power of the downlink channel, increasing the transmit power of the downlink channel if a hypothetically incremented value of the transmit power is lower than the nominal upper limit, increasing the transmit power if total transmit power of the downlink channels is lower than a specified threshold value even when the hypothetically incremented value is greater than the nominal upper limit, and setting the transmit power equal to the nominal upper limit if the hypothetically incremented value is greater than the nominal upper limit and the total transmit power is equal to or higher than the specified threshold value. 
   According to a further specific aspect, the present invention provides a method of controlling the transmit power of a plurality of CDMA downlink channels from a base station within a control range between a nominal lower limit and a nominal upper limit, comprising the steps of receiving, at the base station, a command signal from a mobile station requesting the base station to decrease the transmit power of a downlink channel, decreasing the transmit power of the downlink channel if a hypothetically decremented value of the transmit power is higher than the nominal lower limit, incrementing a count value as long as the hypothetically decremented value is lower than the nominal lower limit, setting the transmit power of the downlink channel to the nominal lower limit if the count value is smaller than a predetermined count value, and decreasing the transmit power of the downlink channel if the count value reaches the predetermined count value. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in further detail with reference to the accompanying drawings, in which: 
       FIG. 1  is a block diagram of a CDMA cell-site base station of the present invention; 
       FIG. 2  is a flowchart of the operation of the transmit power controller of  FIG. 1  according to one embodiment of the present invention; 
       FIG. 3  is a flowchart of an interrupt routine; and 
       FIG. 4  is a flowchart of the operation of the transmit power controller according to a modified embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Referring now to  FIG. 1 , there is shown a CDMA (code division multiple access) cell-site base station of the present invention. The base station is comprised of a plurality of CDMA modems  14 - 1  through  14 -N provided in number corresponding to the number of wireless channels allocated to the base station. The base station includes an antenna  10 , a duplexer  11 , an uplink RF amplifier  12  and a downlink RF amplifier  13 , which form a common antenna system shared by all modems  14 . The cell-site station is connected to a base station controller of the mobile network (not shown) via a line interface  20  that interfaces between the modems  14  and a system controller  21 . A total power detector  22  is provided to detect the total power of downlink transmissions from the base station by summing the transmit power levels of all modems. 
   Each CDMA modem  14  includes a down-converter  15 , an uplink signal processor  16 , a downlink signal processor  17 , a transmit power controller  18  and an up-converter  19 . 
   The base station operates with the antenna  10  to establish CDMA channels. Uplink spread spectrum signals from mobile stations contain control information such as SIR (signal to interference ratio) and TPC (transmit power control) codes produced by the mobile stations. The mobile-transmitted signals, detected by antenna  10 , pass through the duplexer  11  to the RF amplifier  12 . After the RF amplification, the signals are supplied to the down-converter  15  where the radio frequency signals are converted to IF (intermediate frequency) signals or baseband signals. The output of down-converter  15  is fed to the uplink signal processor  16 , which includes a circuit for despeading the signal from a mobile station that uses the same pseudonoise code as that of the modem in the uplink direction and for detecting the transmitted SIR and TPC codes contained in the transmitted signal as well as a control signal necessary for call processing. The SIR and TPC codes detected by the signal processor  16  are supplied to the transmit power controller  18  and the call processing signal is applied to the system controller  21 . The uplink traffic signal of the mobile station is supplied from the signal processor  16  to the line interface  20  and transmitted to the network. 
   Downlink signals from the network are respectively coupled to the modems  14  by the line interface  20 . Downlink signal processor  17  processes the downlink signal by spreading it with a pseudonoise code determined by the system controller  21  to produce a downlink spread spectrum signal. The power level of the downlink spread spectrum signal is controlled by the transmit power controller  18 . The power-control signal is converted to a downlink radio frequency in an up-converter  19 , power-amplified by the RF amplifier  13  and transmitted from the antenna  10 . 
   As will be described in detail, the transmit power controller  18  determines the transmit power of the modem based on the SIR (signal to interference ratio) and TPC (transmit power control) values from the uplink signal processor  16  and the current total power level of the base station supplied from the total power detector  22 . 
   In a first embodiment of the present invention, the transmit power controller  18  operates according to the flowchart of FIG.  2 . 
   When SIR and TPC codes of a given mobile station are detected and supplied from the uplink signal processor  16 , the operation of the controller  18  begins with decision step  31  to check to see if TPC is a “0” or a “1”. 
   If TPC=0, it is determined that the downlink channel of the given mobile station is of excellent quality, requesting that the power level of that channel be decremented, and flow proceeds to decision step  32 . In this step, the transmit power controller  18  calculates the difference in decibel (dB) between the current base-station power level P TX  and a stepsize power value P STP  and determines whether the difference is equal to or greater than the minimum power level P MIN  of the controllable range of the base station. If the decision at step  32  is affirmative, flow proceeds to step  33  to decrement the power level P TX  by the stepsize value P STP  and returns to the starting point of the routine. If the decision at step  32  is negative, flow proceeds to step  34  to compare the SIR value with a predetermined threshold value T SIR . 
   If SIR≧T SIR , it is determined that because of the fact that the downlink channel of the given mobile station is of excellent quality the transmit power of the base station can be lowered below the minimum level P MIN . In other words, the downlink channel still has an excellent quality to tolerate a reduction of power. If this is the case, flow proceeds from step  34  to step  33  to decrement the current transmit power lever P TX  by the stepsize value P STP . 
   If SIR&lt;T SIR , it is determined that a power reduction of the downlink channel would cause a quality degradation. In this case, flow proceeds to step  35  to set the current power level P TX  equal to the minimum level P MIN , and returns to the starting point of the routine. 
   If TPC=1 (step  31 ), it is determined that the downlink channel of the given mobile station is of poor quality, requesting that the power level of that channel be incremented. In this case, flow proceeds to decision step  36 , where the transmit power controller  18  calculates a sum (dB) of the current base-station power level P TX  and the stepsize value P STP  and determines whether the calculated sum is equal to or smaller than the maximum power level P MAX  of the controllable power range of the base station. 
   If the decision at step  36  is affirmative, flow proceeds to step  37  to determine if the current transmit power level P TX  is lower than the minimum power level P MIN . Such a lower-than-minimum situation can occur if the controller  18  has previously executed step  33  following an affirmative decision at step  34 . If this is the case, the controller  18  proceeds from step  37  to step  38  to calculate a sum of minimum power level P MIN  and the stepsize value P STP  and set the current power level P TX  equal to the sum P MIN +P STP , and returns to the starting point of the routine. 
   If the decision at step  37  reveals that a higher-than-minimum situation exists, flow proceeds to step  39  to increment the power level P TX  by the stepsize value P STP  and then returns to the starting point of the routine. 
   If the decision at step  36  is negative, the controller  18  compare the output signal from the total power detector  22  with a threshold value T TOTAL  (step  40 ). If the current total power P TOTAL  is equal to or lower than the threshold value T TOTAL , it is determined that the base station has a sufficient amount of margin to increase the power level of the downlink channel without causing interference with other mobile stations. If this is the case, the controller  18  proceeds to step  39  to increment the current power level P TX  by the stepsize value P STP . 
   If the decision at step  40  is negative, flow proceeds to step  41  to set the current power level equal to the maximum power level P MAX  and returns to the starting point of the routine. 
   While mention has been made of an embodiment in which the incremental stepsize is of constant value, the present invention could equally be as well applied to an embodiment in which the stepsize is adaptively controlled in an interrupt routine as shown in FIG.  3 . 
   In  FIG. 3 , the interrupt routine begins with initialization step  51  in which the controller  18  sets a count value C to 0, and determines, at step  52 , if the TPC value of a downlink channel is “1”, requesting the base station to increase its power level. If so, the controller  18  proceeds to step  53  to check to see if the current power level P TX  of the downlink channel is lower than a threshold level P A . If P TX  is smaller than P A , the controller  18  proceeds to step  54  to increment the count value C by one and compares the count value C to a threshold value C H  at step  55 . If the count value C is smaller than the threshold value C H , steps  52  to  54  are repeated until the count value C exceeds the threshold value C H . If such a lower-than-threshold (P TX &lt;P A ) condition continues for an interval corresponding to the threshold value C H , the controller  18  proceeds from step  55  to step  56  to increment the stepsize value P STP  by P B , where P TX &lt;P B ≦P A . Following step  56 , the transmit power controller  18  returns to the main routine. If the decision at steps  52  and  53  is negative, the controller  18  returns the main routine without altering the stepsize P STP . 
   A modified control algorithm of the transmit power controller  18  is shown in  FIG. 4  in which parts corresponding in significance to those of  FIG. 2  are marked with the same numerals as those used in FIG.  2 . According to this modification, the SIR signal is not used. Instead, a count value K is employed to represent the length of time in which the decremented power level is lower than the lower limit P MIN  of the power control range. 
   In  FIG. 4 , if TPC=0 at step  31 , the downlink channel of a given mobile station is requesting the base station to decrease its power level. Transmit power controller  18  thus proceeds to step  32  to determine whether the difference between P TX  and P STP  is equal to or greater than the minimum power level P MIN  of the base-station power control range. If the decision at step  32  is affirmative, flow proceeds to step  61  to set a count value K to 0 and decrements the power level P TX  by the stepsize value P STP  (step  33 ) and returns to the starting point of the routine. 
   If the decision at step  32  is negative, the count value K is incremented by one (step  62 ) and compared to a threshold value T K  (step  63 ). Thus, the count value K represents the length of time that a situation P TX −P STP &lt;P MIN  continues. If K=T K , the count value K is reset to 0 (step  61 ) and step  33  is executed by decreasing the P TX  value by the stepsize P STP . If K&lt;T K , flow proceeds from step  63  to step  35  to set the current value P TX  to P MIN . As a result, the power level P TX  will be maintained at P MIN  as long as the situation P TX −P STP &lt;P MIN  continues for an interval of time that corresponds to the threshold T K . 
   Therefore, when the decision at step  63  is affirmative, it is determined that despite the fact that the transmit power of a given downlink channel has been held at minimum P MIN  for an extended period of time, the quality of that given channel is still excellent to tolerate a further reduction of power. For this reason, the controller  18  proceeds to step  33  to further reduce the current transmit power level after resetting the K value to zero at step  61 . 
   If TPC=1 at step  31 , indicating that the mobile station is requesting a power increase, the controller  18  proceeds to step  64  to reset the count value K to zero before proceeding to decision step  36 .