Abstract:
A method of calibrating a device under test (DUT) to communicate wirelessly includes providing predetermined reference signal strength values corresponding to a reference device, the reference signal strength values including a first group of signal strength values measured at each of a first plurality of transmission power levels. The method further includes measuring signal strength values for the DUT including a second group of signal strength values measured at each of a second plurality of transmission power levels, mapping the measured signal strength values in the second group of signal strength values to corresponding reference signal strength values in the first group of signal strength values to create a plurality of mapped data pairs, and generating a lookup table according to the mapped data pairs and storing the generated lookup table in a memory of the DUT. The method also includes calibrating the DUT according to the lookup table.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to calibrating a device under test (DUT) to communicate in wireless networks, and more particularly, to a method for calibrating a DUT in order to map signal strength data measured for the DUT to reference signal strength data measured for a reference device. 
     2. Description of the Prior Art 
     In wireless telecommunications, a received signal strength indicator (RSSI) is a measurement of the power level in a received radio signal. The RSSI is a key measurement that is used in a variety of applications such as inter-access point handover strategies, rate adaptation, and location sensing. In general, the higher the RSSI value of a received signal is, the more likely it is that the received signal will be correctly received. Many applications are based on the accuracy of the RSSI. 
     Please refer to  FIG. 1 .  FIG. 1  illustrates a conventional operating environment in which communication units (CU)  20 ,  22 ,  24 ,  26  attempt to wirelessly communicate with an access point (AP)  10 . To communicate with each other through the access point  10 , communication units  20 ,  22 ,  24 ,  26  should first join a basic service set (BSS) mandated by the access point  10 . 
     The relationship between RSSI and dependent variables in the applications can be predetermined if RSSI is calibrated for the communication units  20 ,  22 ,  24 ,  26 . End users can use these applications without an additional training procedure. This will increase the usefulness and practicability of the applications. 
     Please refer to  FIG. 2 .  FIG. 2  is a chart showing a relationship between transmission power level and RSSI according to the prior art. The existing calibration process is based on the relationship between the transmission power level and the received RSSI value of signals received at that corresponding transmission power level. In  FIG. 2  the x-axis (horizontal axis) represents the transmission power level measured in dBm and the y-axis (vertical axis) represents the RSSI value of the received signal in a reference communication unit. 
     Wireless signals are transmitted to the reference communication unit at a variety of different power levels, and the corresponding RSSI values are obtained. Using the (power level, RSSI value) pairs, a nominal curve  30  of the reference communication unit is constructed, as shown in  FIG. 2 . 
     In the typical calibration process for a device under test (DUT), an unadjusted curve of the DUT is obtained by applying the same procedure of obtaining (power level, RSSI value) pairs for several different power levels. Now there are two curves existing in the same coordination system, the nominal curve  30  taken from the reference communication unit, and the unadjusted curve taken from the DUT, and each curve shows RSSI values versus the corresponding power level values. The calibration process involves trying to match the unadjusted curve for the DUT with the nominal curve  30  for the reference communication unit by changing the offset and slope, etc. of the unadjusted curve in order to produce an adjusted curve that matches the nominal curve  30  as closely as possible. 
     However, due to differences between the shapes of the nominal curve  30  and the unadjusted curve, the adjusted curve cannot always be well matched with the nominal curve  30  by simply changing the offset or slope of the unadjusted curve. Therefore, the RSSI values corresponding to some transmission power levels of the adjusted curve cannot always be well calibrated due to the differences in the (power level, RSSI value) pairs between the adjusted curve and the nominal curve  30 . As a result, there is a need for an improved calibration method to more accurately calibrate a DUT in order to map signal strength data measured for the DUT to reference signal strength data measured for a reference communication unit for given transmission power levels. 
     SUMMARY OF THE INVENTION 
     It is therefore one of the primary objectives of the claimed invention to provide methods of calibrating a device under test in order to perform more accurate calibration. 
     According to an exemplary embodiment of the claimed invention, a method of calibrating a device under test (DUT) to communicate wirelessly is disclosed. The method includes providing predetermined reference signal strength values corresponding to a reference device, the reference signal strength values including a first group of signal strength values measured at each of a first plurality of transmission power levels. The method further includes measuring signal strength values for the DUT including a second group of signal strength values measured at each of a second plurality of transmission power levels, mapping the measured signal strength values in the second group of signal strength values to corresponding reference signal strength values in the first group of signal strength values to create a plurality of mapped data pairs, and generating a lookup table according to the mapped data pairs and storing the generated lookup table in a memory of the DUT. The method also includes calibrating the DUT by replacing the measured signal strength values in the second group of signal strength values with the corresponding reference signal strength values in the first group of signal strength values. 
     According to another exemplary embodiment of the claimed invention, a method of calibrating a device under test (DUT) to communicate wirelessly is disclosed. The method includes measuring reference signal strength values corresponding to a reference device, the reference signal strength values including a first group of signal strength values measured at each of a first plurality of transmission power levels. The method further includes measuring signal strength values for the DUT including a second group of signal strength values measured at each of a second plurality of transmission power levels, mapping the measured signal strength values in the second group of signal strength values to corresponding reference signal strength values in the first group of signal strength values to create a plurality of mapped data pairs, and generating a lookup table according to the mapped data pairs and storing the generated lookup table in a memory of the DUT. 
     It is an advantage that the present invention maps measured signal strength values for the DUT to corresponding reference signal strength values for the reference device for given transmission power levels in order to accurately calibrate the DUT for each measured transmission power level. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a conventional operating environment in which communication units attempt to wirelessly communicate with an access point. 
         FIG. 2  is a chart showing a relationship between transmission power level and RSSI according to the prior art. 
         FIG. 3  is a functional block diagram of a device under test according to the present invention. 
         FIG. 4  is a flowchart illustrating a calibration method according to the present invention. 
         FIG. 5  is a chart showing a relationship between transmission power level measured RSSI values for a device under test according to the present invention. 
         FIG. 6  is a chart showing a relationship between the unadjusted RSSI values measured by the DUT and the nominal RSSI values provided from the reference communication unit. 
         FIG. 7  is a chart showing the use of interpolation to calculate additional data points for the nominal RSSI vs. unadjusted RSSI relationship. 
     
    
    
     DETAILED DESCRIPTION 
     Please refer to  FIG. 3 .  FIG. 3  is a functional block diagram of a device under test (DUT)  35  according to the present invention. The DUT  35  comprises a receiver  36  for receiving wireless signals through an antenna  38 . The receiver  36  contains an RSSI module  37  for measuring the RSSI value of received signals. A processor  44  controls the operation of the DUT  35 . A memory  40  is provided for storing a lookup table (LUT)  42  after the DUT  35  has been calibrated. The memory  40  is preferably an electrically erasable programmable read-only Memory (EEPROM) or another form of non-volatile memory such as flash memory. The lookup table  42  stores the calibration data for the DUT  35  in order to allow the calibration data to be quickly retrieved without additional calculations needing to be performed each time the calibration data is to be accessed. As the calibration data will be needed over the lifetime of the DUT  35 , the memory  40  should preferably be a form of non-volatile memory in order to prevent the lookup table  42  from being erased when the DUT  35  loses supplied power. 
     The present invention proposes a new method of calibrating the DUT  35  by adjusting the value measured by the RSSI module  37  with the RSSI values measured for the reference communication unit. Please refer to  FIG. 4 .  FIG. 4  is a flowchart illustrating a calibration method according to the present invention. Steps contained in the flowchart will be explained below. 
     Step  50 : Start the calibration procedure for the DUT  35 . 
     Step  52 : Obtain the nominal curve  30  of the reference communication unit containing (power level, RSSI value) pairs, as shown in  FIG. 2 . This nominal curve  30  can either be directly measured by performing measurements with the reference communication unit, or the nominal curve  30  can be obtained from another source, such as the manufacturer or the seller of the reference communication unit. When creating the nominal curve  30 , the nominal RSSI values corresponding to different transmission power levels are first obtained for the reference communication unit. During this process, the antenna of the reference communication unit is connected with a power transmitter, and the transmission power level of the power transmitter is set to a specific value each time. Then, the RSSI value received by the reference communication unit is measured and is recorded along with the corresponding transmission power level of the power transmitter. The transmission power level of the power transmitter is traversed from minimum to maximum values, P min  to P max , with a small step interval between each successive transmission power level, such as 1 dBm. Next, the RSSI vs. power level nominal curve shown in  FIG. 2  is acquired. Please note that the step interval between successive transmission power levels shown in  FIG. 2  is greater than 1 dBm for enhancing the clarity of  FIG. 2 . 
     Step  54 : Decide how many measurements should be made for the DUT  35 . The number of measurements needed to be performed on the DUT can typically be determined by observing the nominal curve  30  produced in step  52  and shown in  FIG. 2 . Depending on the characteristics of the nominal curve  30 , the step interval for the measurements performed on the DUT  35  is decided. Typically a step interval of 5 dBm between successive transmission power level measurements is used, as compared to the relatively finer step interval of 1 dBm used for the nominal curve  30 . If P min  is set to −95 dBm and P max  is set to −35 dBm, then the number of measurements to be performed is equal to the absolute value of (−95−−35)/5 plus one additional measurement, which in this example is equal to 12+1 for a total of 13 measurements to be performed at 13 different transmission power levels. In general, the number of measurements to be performed can be expressed according to equation (1) as follows: 
     
       
         
           
             
               
                 
                   N 
                   = 
                   
                     
                        
                       
                         
                           
                             P 
                             max 
                           
                           - 
                           
                             P 
                             min 
                           
                         
                         S 
                       
                        
                     
                     + 
                     1 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where N represents the number of measurements performed, P max  represents the maximum transmission power level used in the measurements, P min  represents the minimum transmission power level used in the measurements, and S represents the step interval between each successive transmission power level used in the measurements. 
     Step  56 : Produce the measurements for the DUT  35 . As determined in step  54 , N number of measurements are produced by connecting the DUT  35  to the power transmitter and adjusting the transmission power level of the transmitter from P min  to P max  and with a step interval of S dBm between each successive transmission power level. For each transmission power level used, the corresponding RSSI value is measured using the RSSI module  37  of the receiver  36  in the DUT  35 . Next, an unadjusted curve  80  is produced, as shown in  FIG. 5 . The unadjusted curve  80  is plotted using the measured RSSI values for each of the transmitted power levels. 
     Step  58 : A regression model or other similar model is built to relate the unadjusted RSSI values measured by the DUT  35  to the nominal RSSI values provided from the reference communication unit. As shown in  FIG. 6 , the unadjusted RSSI values measured by the DUT  35  are mapped to the nominal RSSI values provided from the reference communication unit to produce a nominal RSSI vs. unadjusted RSSI relationship. In the relationship shown in  FIG. 6 , the unadjusted RSSI values are adapted directly from the measurements produced in step  56 , and the unadjusted RSSI values are mapped to the nominal RSSI values having the same corresponding transmission power levels to produce mapped data pairs. Using these mapped data pairs, a regression model y≈f(x,β) of the unadjusted RSSI curve vs. nominal RSSI relationship is constructed, where β represents unknown parameters, independent variable x represents the unadjusted RSSI values, and dependent variable y represents the nominal RSSI values. Linear regression can also be used to build the regression model, and interpolation or extrapolation of the mapped data points can be used to calculate approximate values of additional data points. As shown in  FIG. 7  the nominal RSSI vs. unadjusted RSSI relationship can be enhanced using interpolation to calculate additional data points that were not included as a result of the initial measurements. For example, if there is a need to find the nominal RSSI value corresponding to an unadjusted RSSI value of  38  and shown as point C in  FIG. 2 , interpolation can be used to calculate this value based on the data associated with existing points A and B. Either linear interpolation, polynomial interpolation, spline interpolation, or other forms of interpolation can be used, according to the data being modeled. Extrapolation can be performed in a similar manner in order to find values existing outside of two existing points. It is also possible to use other methods to model the relationship of the data points, such as artificial intelligence. It should be noted that linear regression is just one method that can be used to build the regression model. Besides the regression model, it is also possible to use interpolation or extrapolation methods to model the relationship. Interpolation or extrapolation of the mapped data points can be used to calculate approximate values of additional data points as shown in  FIG. 7 . 
     Step  60 : Determine whether two or more unadjusted RSSI values in the regression model built in step  58  are equal to one another and are mapped to different nominal RSSI values. In other words, a check is made to determine if any unadjusted RSSI value is mapped to multiple nominal RSSI values in the nominal RSSI vs. unadjusted RSSI relationship. If so, step  64  is executed. If not, step  62  is executed. 
     Step  62 : The lookup table  42  is built according to the regression model built in step  58 . The lookup table  42  stores a series of data pairs in which the unadjusted RSSI values are mapped to the nominal RSSI values having the same corresponding transmission power levels. The lookup table  42  contains (unadjusted RSSI value, nominal RSSI value) data pairs, and is stored in the memory  40  for quick and convenient access. Next, step  66  is executed. 
     Step  64 : Increase the number of measurements that should be made for the DUT  35  and go back to step  56 . The number of measurements is increased to alter the step interval between successive transmission power level measurements. By adjusting the step interval for the DUT  35 , a different set of data pairs will be produced from the mapping of the unadjusted RSSI values measured by the DUT  35  to the nominal RSSI values provided from the reference communication unit. This remapping is performed to avoid any unadjusted RSSI value being mapped to multiple nominal RSSI values in the nominal RSSI vs. unadjusted RSSI relationship. 
     Step  66 : The lookup table  42  is searched for finding a nominal RSSI measurement when inputting an unadjusted RSSI value input to the lookup table  42 . The found nominal RSSI measurement is referred to as an adjusted RSSI value, which is also the calibrated RSSI value for the DUT  35 . Therefore, the unadjusted RSSI value measured by the DUT  35  is instead replaced by the adjusted RSSI value according to the data pairs stored in the lookup table  42 . 
     Step  68 : End. 
     The above calibration method shown in  FIG. 4  is applicable to a wide variety of different types of DUTs. The DUT does not necessarily need to have a receiver and could have a transmitter instead, and other measurements besides RSSI can be used for measuring signal strength values. The DUT can operate in and communicate wirelessly in any type of wireless communication network, such as a mobile phone network or a wireless local area network (WLAN). 
     In summary, the present invention method improves calibration for a DUT by mapping measured signal strength values for the DUT to corresponding reference signal strength values for the reference device for given transmission power levels in order to accurately calibrate the DUT for each measured transmission power level. In this way, the DUT can be more accurately calibrated over all used power transmission levels than was previously possible using prior art techniques. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.