Patent Document

CLAIM OF PRIORITY UNDER 35 U.S.C. §119 
     The present Application for Patent claims priority to Provisional Application No. 60/598,802 entitled “METHOD AND APPARATUS FOR EXCESS CAPACITY IN A WIRELESS NETWORK” filed Aug. 3, 2004, and assigned to the assignee hereof and hereby expressly incorporated by reference herein. 
     CLAIM OF PRIORITY UNDER 35 U.S.C. §120 
     The present application for patent is a Continuation and claims priority to patent application Ser. No. 11/195,877 entitled “Estimation of received signal strength” filed Aug. 2, 2005 now allowed, and assigned to the assignee hereof and hereby expressly incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field 
     The present invention relates generally to wireless telephone communications, and specifically to measurement of received signal strength. 
     2. Background 
     One of the parameters that is important to determine for efficient operation of a base-station transceiver system (BTS) in a cellular telephone network is the reverse link excess capacity of the BTS. The reverse link excess capacity is measured in terms of a theoretical maximum number of users of the BTS, which in turn is a function of the energy received by the receiving system of the BTS compared with a noise figure of the receiving system. Accurate measurement of this energy difference is difficult to perform. 
     Methods are known in the cellular network art for determining the noise figure of the receiving system, which is a function of the inherent thermal noise as well as a noise contribution from the receiving system itself. The noise contribution from the receiving system is based on the inherent thermal noise and the gain (or loss) of the elements in the receiving system. For example, the noise contribution of each of the elements in the receiving system at the BTS, from the antenna to the final detector of the system, may be evaluated, and combined with the inherent thermal noise. However, while determination of noise contributions and gain of passive components in the receiving system is relatively straightforward, and the values do not change over time, this is typically not the case for active components. Determination of the noise contribution and gain from active components is usually more complicated and time-consuming; in addition, the noise contribution of active components typically changes over time, and such change may be difficult to predict. Furthermore, and adding to the complication, the noise figure of the receiving system is typically frequency and part dependent, as well as varying with temperature. 
     Measurement of the energy received by the receiving system at the detector typically suffers from the same problems mentioned above, namely, gain variation with temperature, frequency, and time. 
     An alternative method for measuring the relative noise level of the receiving system, known in the art, is to arrange that all mobile transceivers transmitting to a BTS are simultaneously silent for a short time period, during which the noise at the BTS may be measured. This method has the advantage, compared to the method described above, of being operable in an active system, at the cost of a reduction of resources during the silent period and the complexity of reacquiring the mobile transceiver signals. In addition, the method suffers from the fact that the BTS can only silence mobile transceivers it controls, and cannot prevent other transmissions from reaching the BTS, unless all BTSs are synchronized and perform the silencing at the same time. 
     There is thus a need for an improved method for measuring the noise figure of a receiving system in a BTS, for the purposes of measuring the reverse link excess capacity of the BTS. 
     SUMMARY OF THE INVENTION 
     In an embodiment of the present invention, a control unit of a base-station transceiver system (BTS) determines a reverse link excess capacity of the BTS. The reverse link excess capacity is typically determined in terms of a number of users that are able to place new calls via reverse link signals to the BTS. The control unit determines the excess capacity by successive measurements of strengths of signals received by a receiving system of the BTS. The control unit analyzes the measurements to find a minimum signal strength received by the receiving system, and the minimum signal strength is used to give an approximate noise level of the receiving system. In calculating the approximate noise level, the control unit periodically adds an “aging value” to the approximate noise level, and then continues its analysis of signal strengths to update the minimum signal strength. Adding the aging value simulates aging, and the consequent rise in noise level, of the receiving system. The reverse link excess capacity of the BTS at any instant may be calculated by comparing the approximate noise level, determined as described above, with the actual received signal measured at the receiving system at that instant. 
     Using the minimum signal strength as the approximate noise level of the receiving system is a simple and efficient way to estimate the noise of the receiving system. The inventors have found that the results are comparable with those of more complicated, time-consuming, and costly systems for estimating the noise level, and give good results for the determination of the reverse link excess capacity. 
     In one embodiment, determining the size of the reverse link resources includes performing, subsequent to the third measurement, a fourth measurement of a fourth signal strength received at the BTS, and forming a further comparison between the fourth measurement and the minimum received signal strength. Typically, determining the size of the reverse link resources includes determining a reverse link excess capacity of the BTS in response to the further comparison. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a cellular network system, according to an embodiment of the present invention; 
         FIG. 2  illustrates a power management relationship, according to an embodiment of the present invention; 
         FIG. 3  is a schematic graph of a received signal energy vs. a number of users, according to an embodiment of the present invention; 
         FIG. 4  is a flowchart of a process performed by a control unit in the system of  FIG. 1 , according to an embodiment of the present invention; and 
         FIG. 5  is a flowchart of another process performed by the control unit, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Reference is now made to  FIG. 1 , which is a schematic diagram of a cellular network system  10 , according to an embodiment of the present invention. Hereinbelow, by way of example, system  10  is assumed to operate as a code division multiple access (CDMA) network. However, the principles of the present invention are not limited to any particular type of network, so that system  10  may be a network operating under any cellular network system known in the art, such as a frequency hopping spread spectrum (FHSS) system, an orthogonal frequency division multiple access (OFDMA) system, or a combination of these and/or CDMA systems and/or other cellular network systems. System  10  comprises one or more base-station transceiver systems (BTSs), only one of which, BTS  18 , is illustrated in  FIG. 1  for clarity. BTS  18  is coupled to the rest of network  10  via a base-station controller (BSC)  22 , and a mobile switching center (MSC)  38 . 
     BTS  18  is operated by a control unit  20 , which is typically physically located at the BTS, although the control unit may be located at any convenient location in system  10 . During the course of operation of BTS  18 , control unit  20 , on a substantially continuous basis, estimates a reverse link excess capacity of the BTS. The method of estimation is described in more detail hereinbelow. Control unit  20  then uses the excess capacity estimation for admission control, wherein the control unit decides if the BTS has sufficient resources to admit a new incoming call, and/or if additional reverse link resources need to be allocated to existing users of the BTS. The resources typically comprise channels which may be operated at different bit rates. In a network where voice comprises the majority of the traffic, each user is typically allocated one channel, so that the number of users and the number of channels allocated by the control unit are substantially equal. In a network where there are other forms of traffic, such as data and/or video transfer, the number of channels is typically larger than the number of users. 
     BTS  18  operates a sector, herein termed a coverage area  14 , within which generally similar mobile transceivers  12  are able to transmit signals to BTS  18 , and receive signals from the BTS, via one or more antennas coupled to the BTS, the transceivers acting as users of the BTS. Except where otherwise stated below, the following description assumes that BTS  18  has only one antenna  16 . BTS  18  thus comprises a receiving system  24  receiving reverse link signals from the mobiles, and a transmission system  26  transmitting forward link signals to the mobiles. Control unit  20  operates both systems. The transmission system receives forward link signals from BSC  22  at a transmission system input port  34 , and outputs amplified forward link signals from a transmission system output port  36  to antenna  16 . 
     Receiving system  24  receives signals from antenna  16  at an input receiving system port  30 , and outputs amplified, filtered, and detected signals from an output receiving system port  32 . Receiving system  24  also provides control unit  20  with an indication of the power received by antenna  16 , typically by measuring the levels at one or more detector stages of the receiving system. The indication of the received power at the antenna input, assumed herein to be measured in dBm, is herein termed the received signal strength indication (RSSI_dBm). 
     To determine the reverse link excess capacity, control unit  20  needs to know the theoretical reverse link capacity of BTS  18 , as well as the reverse link resources that are in use. As is explained in more detail below with reference to  FIG. 3 , the theoretical reverse link capacity of the BTS depends on the difference between the strength of the received signals and a noise figure of the receiving system of the BTS. 
       FIG. 2  illustrates a power management relationship  50  between a number of modules operated by control unit  20  for the determination of the reverse link excess capacity, according to an embodiment of the present invention. A receiver noise figure setting module  52  generates a theoretical value, RX_NOISE, of the noise of receiving system  24 . RX_NOISE is a sum of the thermal noise of the receiving system, substantially determined by the bandwidth and operating temperature of the receiving system, with an estimate of the noise contribution of the components of the receiver system. RX_NOISE may be altered by control unit  20  for purposes of link balancing, as is known in the art. RX_NOISE is transferred to a receive power correction value module  54 , which uses the value of RX_NOISE to determine a corrected value, RSSI_CORRECT_VAL, of RSSI_dBm. The operation of correction value module  54  is described in more detail with respect to  FIG. 4  below. 
     Control unit  20  operates a receive power estimation module  58  to generate the value RSSI_dBm, using, as described above, one or more detected levels in receiving system  24 . Control unit  20  also operates a reverse link excess capacity estimation module  56  to determine an estimate of the excess capacity, using the values of RSSI_dBm, RSSI_CORRECT_VAL, and RX_NOISE. 
     Other elements of relationship  50 , and shown in  FIG. 2 , are described below as required. 
       FIG. 3  is a schematic graph  70  of a received signal energy at BTS  18  vs. a number of equal-power users, according to an embodiment of the present invention. The horizontal axis of graph  70  shows a theoretical number of users M that are able to transmit to BTS  18 , for a given total energy E received by receiving system  24  of the BTS. Graph  70  is based on the pole capacity equation which is known in the art. 
     In graph  70 , line  76  determines a maximum theoretical number of equal-power users Mmax in a CDMA system, requiring infinite receive energy at BTS  18 . Typically one would have a reasonable backoff from Mmax to allow for a practical trade-off between the number of users and the receive energy, which in turn contributes to system stability. 
     In order to implement this backoff, control unit  20  sets a threshold M 70 , which translates into a receive power RSSI_thresh. A typical value for M 70  is 80% of Mmax, which translates to an RSSI_thresh of 7 dB above a receiving system noise energy Nsys. Nsys depends on the thermal noise energy N 0  and the noise generated by the receiving system itself. 
     While the shape of graphs such as graph  70  is substantially invariant between different receiving systems of different BTSs, its vertical axis intercept Ev depends upon the gain characteristics of the specific receiving system. Thus, as illustrated schematically in  FIG. 3 , two other receiving systems may have graphs  72  and  74 , having substantially similar shapes to graph  70 , but each having different vertical axis intercepts, herein termed E 70 , E 72 , and E 74 . A BTS with a measurement offset of the receive energy above Nsys, as characterized by graph  72  or graph  74 , will cause control unit  20  to limit the number of users to M 72  and M 74  instead of the intended value M 70 . For example, for a value M 70  that is 80% of Mmax, a +/−3 dB error in the receive energy above Nsys will result in M 72  and M 74  being 60% and 90% of Mmax respectively. 
     The Background of the Invention describes some prior art systems for measuring the value of the receive energy above Nsys; in the instant specification we describe a method for estimation of the value of the receive energy above Nsys which does not rely on time-consuming and expensive calibration of the receiving system, or on enforced quiet periods of mobiles transmitting to BTS, such as those used in the prior art. The inventors have found that the method, described in reference to  FIG. 4  below, gives good results for the effective estimation of the value of the receive energy above Nsys, and thus for the reverse link excess capacity of BTS  18 , without the drawbacks present in the prior art. 
       FIG. 4  is a flowchart of a process  90  performed by control unit  20 , according to an embodiment of the present invention. Control unit  20  operates process  90 , in receive power correction value module  54  ( FIG. 2 ), periodically, typically every 20 ms, in order to determine a corrected value, RSSI_CORRECT_VAL, of the RSSI_dBm value output by receiving system  24 . The control unit effectively uses this corrected value as the vertical axis intercept E 70  ( FIG. 3 ). 
     In an initialization step  92  of process  90 , control unit  20  sets initial values of variables used in the process. Thus, unit  20  initially sets RSSI_CORRECT_VAL to be equal to 0, and the unit sets a range factor RSSI_CORRECT_RANGE, which limits the values of RSSI_CORRECT_VAL output by process  90  to be within a pre-determined range equal to ±RSSI_CORRECT_RANGE; RSSI_CORRECT_RANGE is typically approximately 5 dB. At initialization unit  20  also sets an offset, RSSI_CORRECT_OFFSET, that the unit uses as a correction factor in evaluating RSSI_CORRECT_VAL; RSSI_CORRECT_OFFSET is typically 0 dB. The offset may be set to be non-zero, for example when there is no expectation of a substantially zero load. 
     In a second step  94 , unit  20  checks if BST  18  has been subject to blossoming or wilting within a preset period. (The terms blossoming and wilting are known in the art, and refer to coming on-line, or going off-line, of a base-station transceiver system.) The preset period is typically of the order of 30 seconds, although any other suitable preset period may be used. If blossoming or wilting have not occurred in the preset period, process  90  continues to a third step  96 ; if they have occurred, the process waits until the preset period has completed before continuing to step  96 . 
     In step  96 , control unit  20  reads a most recent value of the receiver system received power, RSSI_dBm, and finds the minimum of the most recent value and a previous value of RSSI_dBm. The previous value was read by control unit  20  in a previous time period during which the control unit operated process  90 . The minimum that is determined in step  96  is herein termed RSSI_REF_FILT. 
     In a fourth step  98 , the corrected value of RSSI_dBm, RSSI_CORRECT_VAL is evaluated by control unit  20  according to equation (1) below. Equation (1) takes the minimum value of step  96 , and uses it to correct the theoretical receiver system noise RX_NOISE.
 
RSSI_CORRECT_VAL=RX_NOISE−(RSSI_REF_FILT+RSSI_CORRECT_OFFSET)  (1)
 
     Also in step  98 , unit  20  verifies that the result of applying equation (1) does not set the value of RSSI_CORRECT_VAL to be outside the range of acceptable values defined by RSSI_CORRECT_RANGE. If equation (1) does give a value outside the range, unit  20  alters the value to be at the appropriate limit of the range. 
     The value of RSSI_CORRECT_VAL is then transferred to reverse link excess capacity estimation module  56 , which uses the value, as described with reference to  FIG. 5  below, to determine the excess capacity of BTS  18 . 
     In a last step  100  of process  90 , control unit  20  periodically adds an “aging factor” to the evaluated value of RSSI_REF_FILT, according to equation (2) below:
 
RSSI_REF_FILT=RSSI_REF_FILT+AGE_FACTOR   (2)
 
     Control unit  20  typically generates the periodicity, AGE_PERIOD, for applying equation (2), and the value of the aging factor, AGE_FACTOR, in initial step  92  of process  90 . Typical values for AGE_PERIOD and AGE_FACTOR are of the order of 1 hour and approximately 0.1 dB, respectively. 
     After step  100 , process  90  returns to the beginning of step  94 . 
     It will be understood from inspection of process  90  that the value RSSI_CORRECT_VAL is a function of the minimum value of RSSI_dBm, determined over the time during which process  90  is operated. It will also be understood that RSSI_CORRECT_VAL approximates the difference between the vertical axis intercept Ev, illustrated in  FIG. 3 , and the noise value Nsys of receiving system  24  described above. 
     The aging factor AGE_FACTOR simulates the change in noise value of receiving system  24  over time, so that the exemplary values given above increase the noise value of the receiving system, with no other change being input to the system, by 0.1 dB every hour. However, it will be understood that this change may be overridden by a smaller actual noise value, RSSI_dBm, received by the receiving system. 
       FIG. 5  is a flowchart of a process  120  performed by control unit  20 , according to an embodiment of the present invention. Control unit  20  operates process  120  periodically, typically with a period of the order of 20 ms, in reverse link excess capacity estimation module  56  ( FIG. 2 ), to determine a reverse link excess capacity, RL_EXCESS_CAP. 
     In a first step  122  of process  120 , unit  20  inputs the value of RX_NOISE from setting module  52 , the value of RSSI_dBm from receive power estimation module  58 , and the value of RSSI_CORRECT_VAL, determined by process  90 , from receive power correction module  54 . 
     In a second step  124 , unit  20  calculates an excess capacity, CRX, for BTS  18  according to an equation (3): 
     
       
         
           
             
               
                 
                   
                     C 
                     RX 
                   
                   = 
                   
                     10 
                     ⁢ 
                     
                       
                         ( 
                         
                           RX_NOISE 
                           - 
                           
                             ( 
                             
                               RSSI_dBm 
                               + 
                               
                                 RSSI_CORRECT 
                                 ⁢ 
                                 _VAL 
                               
                             
                             ) 
                           
                         
                         ) 
                       
                       10 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Typically, the value of CRX is approximately 0.5 (corresponding to a loading of 50%) or approximately 0.25 (corresponding to a loading of 75%), since lower values of CRX, corresponding to higher loading values, may lead to transmitted traffic being perceived to be of sub-optimal quality. 
     In a final step  126 , unit  20  ensures that the calculated value of RL_EXCESS_CAP is within limits of 0% and 100% by applying a limiting equation (4) to the excess value CRX determined in step  128 .
 
 RL _EXCESS_CAP=min( C   RX ;1)×100%   (4)
 
     Returning to  FIG. 1 , antenna  16  may comprise two or more antennas, which each have separate and generally similar receiving systems  24 . In this case, by using a process of diversity known in the art, signal reception in the sector corresponding to area  14  may be improved compared to reception using a single antenna. 
     In the case of two or more antennas  16 , the processes described above may be applied separately to each receiving system of the respective antennas, to estimate a reverse link excess capacity for each of the systems. Typically, the estimation that is used may then be based on the worst of the estimates, or on an average of some or all of the different receiving system estimates. Since the receiving systems are distinct systems, control unit  20  may use the same or different initial values of variables, such as AGE_FACTOR and/or AGE_PERIOD, for each receiving system in implementing the processes described above. 
     It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.

Technology Category: 5