Patent Publication Number: US-2003228846-A1

Title: Method and system for radio-frequency proximity detection using received signal strength variance

Description:
RELATED APPLICATIONS  
     [0001] This application is related to U.S. provisional application Ser. No. 60/386,493, filed Jun. 5, 2002 and from which it claims benefits under 35 U.S.C. §119(e). 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] The present invention relates generally to radio-frequency distance measuring systems, and more specifically, to a method and system for locating a transmitting device using a measured received signal strength variance.  
       [0004] 2. Background of the Invention  
       [0005] Wireless data communications, voice communications and control devices are finding increasing use in home, office and industrial applications. In many “short range” wireless applications, it is essential or at least desirable to know whether two or more devices are in close proximity. Typical applications are secure data transfer, wireless payments, proximity based equipment activation and other applications where the proximity of a device provides a clue as to whether the identity or locale of a transmitter is consistent with expectations of security.  
       [0006] Estimation of distance between two radio devices is traditionally done by measuring either the strength of a received signal, or the propagation delay of a two way signal. The signal strength techniques (Received Signal Strength Indication—RSSI) performance is poor at best in terms of accuracy and reliability, as propagation loss is highly dependent on environmental conditions and the received signal is affected by fading due to multi-path propagation. In addition, distance estimation requires cooperation from a detected party, as the transmitter power is typically unknown for a transmitting device unless the transmitting device communicates the transmitter power to the distance detecting receiver or the receiver transmits the RSSI to the transmitting device.  
       [0007] Further, transmitter power tolerances, antenna gain variations and attenuation due to obstructions such as human bodies degrade the overall reliability of an RSSI-based proximity measurement.  
       [0008] Propagation delay-based distance measurement methods yield accurate and reliable results, but usually require additional circuitry and complexity in both transmitters and receivers. Propagation delay-based distance measurement also typically requires special transmission packets and sophisticated algorithms to compensate for fading and motion of the transmitting and receiving devices.  
       [0009] Therefore, it would be desirable to provide a signal strength-based distance measuring technique and system that have improved performance in the face of fading, system component variations and signal absorption.  
       SUMMARY OF THE INVENTION  
       [0010] The above objectives of providing improved signal strength-based distance measuring are achieved in a method and system. The method is embodied in a receiver system that determines a physical location or distance of a transmitting wireless device by measuring the variance of a received signal over time and comparing the variance to predetermined ranges of variance. A device that falls outside of a predetermined range can be denied service or services can be restricted for that device, providing enhanced security and/or reliability of communications. Variance measurements can be performed at both ends of a transceiver link, and the results exchanged between transceivers, providing improved accuracy of the variance computation.  
       [0011] The foregoing and other objectives, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiment of the invention, as illustrated in the accompanying drawings.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0012]FIG. 1 is a graph depicting variance of signal strength as measured in a system in accordance with an embodiment of the present invention.  
     [0013]FIG. 2 is a block diagram of a location finding unit in accordance with an embodiment of the present invention.  
     [0014]FIG. 3 is a graph depicting operation of multiple location finding units in accordance with an embodiment of the present invention.  
     [0015]FIG. 4 is a flowchart depicting a method in accordance with an embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT  
     [0016] The present invention provides a method and system for determining a proximity distance of a transmitting device by measuring the amplitude of a signal received at a receiver. The amplitude measurement is improved over prior amplitude measurements, with a consequent improvement in estimation accuracy of the distance between the transmitter and receiver. Referring now to FIG. 1, a graph depicting signal strength variance versus distance between transmitter and receiver is depicted. As is evident from the graph, variance of the signal strength increases with separation distance between the transmitter and receiver, and the present invention uses the variance information to provide improved proximity detection. Variance is a statistical quantifier given for finite number of samples n by the formula: 
     var=[ n·Σs   2 −(Σ s   2 )]/[ n ·( n− 1)] 
     [0017] where:  
     [0018] n is the number of samples  
     [0019] s is a recorded signal strength, expressed in linear (volts, watts) or logarithmic (dBm) terms.  
     [0020] The statistical variance of the strength of a received signal at a wireless device is strongly correlated to the distance from the transmitter, particularly in short range (0 to 5 m) communication links. Therefore, determining the variance of the received signal yields a strong indicator of proximity distance that can be used to implement an improved amplitude-based proximity detection system and method.  
     [0021] Referring now to FIG. 2, a pair of transceivers  20 A and  20 B in accordance with an embodiment of the invention and forming a system in accordance with an embodiment of the invention are depicted. Transceivers  20 A and  20 B each comprise a transmitter  22 A and  22 B and a receiver  24 A and  24 B coupled to antennas  21 A and  21 B, whereby the transceivers exchange RF signals carrying voice, data, video or other information. The communications channel may be a discrete channel or a shared mechanism such as Spread Spectrum, including frequency hopping or in a direct sequence system. In general, the system depicted in FIG. 2 applies to wireless devices including receive-only and transmit-only devices, as the present invention can be practiced in a receiver having no transmit capability in response to an external transmitter having no receive capability. Within each of transceivers  20 A and  20 B is a processor/control block  26 A and  26 B, that provides computation and control in accordance with embodiments of the present invention.  
     [0022] Referring now to FIG. 3, details of processor/control block  26 A (and similarly processor/control block  26 B) is shown. Processor/control block  26 A includes a signal strength detector  30  that is coupled to a signal from receiver  24 A that is proportional to received signal strength at receiver  24 A. Signal strength detector is coupled to radio control, which delivers a numeric indication of received signal strength to a processor  34 . Processor  34  is coupled to a memory  36  that contains program instructions for execution by processor  34 , including program instructions for carrying out methods in accordance with an embodiment of the present invention. Radio control  32  is also coupled to a human machine interface (HMI) for providing an interface of transceiver  20 A accessible by a user (e.g., an LCD display and a keypad).  
     [0023] Processor  34  computes the variance of the detected received signal strength provided by radio control  32  and makes determinations of proximity from the variance. The signal strength measurement and is repeated periodically during normal communication and a variance is computed over a predetermined number of samples (100 in this illustration). For example, the variance can be computed every 100 ms on a sample interval of 1 ms. Sampling of the signal strength (RSSI) can be performed on a single channel or on multiple channels (e.g., in frequency hopping systems).  
     [0024] In the illustrative embodiment, radio control  32  in transceiver  20 A sets the receiver  24 A reception frequency, range of frequencies, or channel. Then, radio control  32  provides the output of signal strength detector  30  to processor  34 , which computes the variance over a number of collected signal strength samples. After the variance value is computed, processor  34  compares the variance to predefined limit criteria, and a decision is made whether the detected device is within a certain proximity range. The proximity decision can be used to authorize an operation, (e.g. open a door) or be displayed to a user for further actions. Alternatively, processor  34  can estimate a distance from the variance and limits can be applied to the distance estimation.  
     [0025] The proximity decision can be performed at both ends of a communication link, e.g., transceiver  20 A can compute the signal strength variance of a signal received from transceiver  20 B and transceiver  20 B can compute the signal strength variance of a signal received from transceiver  20 A. Comparison of the results by exchanging variance or proximity data between transceivers  20 A and  20 B yields an improvement in accuracy of the proximity measurement, as the results should ideally be symmetric (i.e., the distance is identical).  
     [0026] Further computation by processor  34  may be performed, The confidence level of the result computed from the variance can be further enhanced by various techniques and algorithms, including majority voting, filtering, and multiple sampling rates computations. Interference immunity can be provided or improved by one or more of several mechanisms. Where the integrity of the received signal is constantly monitored, RSSI readings may be gated to remove noise spikes and/or to remove any readings, when the received error rate exceeds a specific threshold, thus removing spurious samples from the variance computation. Also, where many channels are involved in the communication link such as in frequency hopping systems, channel readings can be skipped if its RSSI or RSSI variances are significantly different from the majority of the other channels, which indicates corruption due to narrow band interference.  
     [0027] Additional variables can also be added to the proximity formula, providing a proximity decision in conformity with a function of RSSI variance, error rate and absolute RSSI. Further, if an indication of transmitter power is sent from the transmitting device (i.e., numeric data corresponding to absolute transmitter power), then absolute path loss and path loss variance can be computed and used to compute the proximity indication.  
     [0028] The methods of the present invention are suitable for short range detection applications, where received signals are normally strong enough to produce a high signal to noise ratio (SNR), which improves the reliability of the results. In cases where the signals are too strong and cause saturation of the RSSI measurement, which is a condition easily detectable by the wireless devices. The saturation indication can be considered a proximity indication overriding the variance decision, or front-end attenuation can be inserted in the receive path to eliminate saturation.  
     [0029] Referring now to FIG. 4, a method in accordance with an embodiment of the present invention is depicted. First, a signal is received from an accessing (transmitting) device (step  40 ). Next, the variance of the received signal is measured over time (step  42 ). If the computed variance is within the expected range for authorized access (decision  44 ), then access is granted (step  46 B). If the computed variance is not within the expected range (decision  44 ), access is denied or restricted (step  48 ). While the method depicted is described in terms of a security-type model, the variance decision-based techniques of the present invention are equally applicable to other proximity detection uses, such as using a proximity indication to validate a communication to avoid or reduce channel errors.  
     [0030] There are many advantages of the techniques of the present invention. One is simple implementation, as most or all of the components necessary to carry out the method of the present invention exist within many wireless devices already, with the exception of program instructions for carrying out the method of the present invention. The above-illustrated techniques are also flexible as the sampling rate, sample population and number of trials may be adjusted and optimized for a particular application. The techniques are also autonomous, requiring no information on transmitting power from the transmitting device in order to implement the basic method of the present invention. The variance is also, only slightly affected by environmental conditions and for known conditions, the detection ranges may be adjusted.  
     [0031] While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form, and details may be made therein without departing from the spirit and scope of the invention.