Method for Locating Mobile Units Based on Received Signal Strength Ratio

Abstract

Location of mobile units in a wireless local area network is based on use of signal strength ratios and other criteria. In one embodiment absolute value of signal strength is used to derive a second value of location. The second value may be used as the location if the system is not calibrated and may also be used to calibrate the system. Alternatively time difference of arrival may be used in combination with signal strength ratio.

1 - 8 . (canceled)  
   
   
       9 . A method for locating mobile units arranged for radio communication with fixed devices, comprising: 
 detecting received signal strength for transmissions from one of said mobile units to a first selected plurality of fixed devices having known locations;    detecting time of arrival for said transmissions at a second selected plurality of said fixed devices;    computing ratio of said received signal strength for at least one pair of said fixed devices in said first plurality;    computing time difference of arrival for at least one pair of said fixed devices in said second plurality; and    calculating the maximum likelihood location for said mobile unit using said computed ratio and said time difference of arrival.    
   
   
       10 . A method as specified in  claim 9 , wherein said first plurality and said second plurality include at least three fixed devices.  
   
   
       11 . A method as specified in  claim 9 , wherein said first plurality is the same as said second plurality and includes at least three fixed devices.  
   
   
       12 . A method as specified in  claim 9 , wherein the mobile units operate according to the IEEE Standard 802.11 communication protocol.  
   
   
       13 . A method as specified in  claim 9 , further comprising: 
 wherein the first plurality of fixed devices includes a first plurality of access points and the second plurality of fixed devices includes a second plurality of access points.    
   
   
       14 . A method as specified in  claim 9 , wherein for the calculating of the maximum likelihood location, a line-of-sight Friis propagation model and an inverse square law of distance apply.  
   
   
       15 . A method as specified in  claim 9 , wherein the calculating includes selecting a value for maximizing a likelihood function.  
   
   
       16 . A method as specified in  claim 9 , wherein the method is performed in an indoor location.  
   
   
       17 . A method as specified in  claim 9 , wherein the calculating is performed based on locations of the first plurality of fixed devices and of the second plurality of fixed devices being known.  
   
   
       18 . A method as specified in  claim 17 , wherein the calculating is performed based on a noise covariance being present and based on an estimate of a quantity related to a power radiated by a transmitter directivity of the mobile unit.  
   
   
       19 . A system for locating mobile units arranged for radio communication with fixed devices, comprising: 
 means for detecting received signal strength for transmissions from one of said mobile units to a first selected plurality of fixed devices having known locations;    means for detecting time of arrival for said transmissions at a second selected plurality of said fixed devices;    means for computing ratio of said received signal strength for at least one pair of said fixed devices in said first plurality;    means for computing time difference of arrival for at least one pair of said fixed devices in said second plurality; and    means for calculating the maximum likelihood location for said mobile unit using said computed ratio and said time difference of arrival.    
   
   
       20 . A system as specified in  claim 19 , wherein said first plurality and said second plurality include at least three fixed devices.  
   
   
       21 . A system as specified in  claim 19 , wherein said first plurality is the same as said second plurality and includes at least three fixed devices.  
   
   
       22 . A system as specified in  claim 19 , further comprising: 
 wherein the first plurality of fixed devices includes a first plurality of access points and the second plurality of fixed devices includes a second plurality of access points.    
   
   
       23 . A system as specified in  claim 19 , wherein for the calculating of the maximum likelihood location, a line-of-sight Friis propagation model and an inverse square law of distance apply.  
   
   
       24 . A system as specified in  claim 19 , wherein the calculating includes selecting a value for maximizing a likelihood function.  
   
   
       25 . A system as specified in  claim 19 , wherein the method is performed in an indoor location.  
   
   
       26 . A system as specified in  claim 19 , wherein the calculating is performed based on locations of the first plurality of fixed devices and of the second plurality of fixed devices being known.  
   
   
       27 . A system as specified in  claim 26 , wherein the calculating is performed based on a noise covariance being present and based on an estimate of a quantity related to a power radiated by a transmitter directivity of the mobile unit.  
   
   
       28 . A device for storing computer instructions that when executed results in a performance of the following: 
 computing a ratio of a detected received signal strength for at least one pair of fixed devices in a first plurality of fixed devices;    computing a time difference of arrival for at least one pair of fixed devices in a second plurality of fixed devices; and    calculating the maximum likelihood location for a mobile unit using said computed ratio and said time difference of arrival.