Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-09-09
2001-05-29
Hunter, Daniel (Department: 2684)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S522000, C455S432300, C455S422100, C455S442000, C455S438000
Reexamination Certificate
active
06240291
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to wireless communication systems. More specifically, it relates to improved handoff techniques in cellular communication networks.
BACKGROUND OF THE INVENTION
Wireless communication networks provide communication services to mobile devices such as cellular telephones, wireless modems, and pagers. A network typically has a set of base stations distributed across a geographical service area. A mobile device accesses the network at any given time through a wireless communication link with one of the base stations. As the device moves through the service area, however, the network must handoff the communication link to other base stations so that the service is not interrupted or degraded. Techniques related to handoff in wireless communication system are disclosed in U.S. Pat. No. 5,920,549 to Bruckert et al., U.S. Pat. No. 5,901,354 to Menich et al., U.S. Pat. No. 5,754,945 to Lin et al., and U.S. Pat. No. 5,657,487 to Doner. These patents are incorporated herein by reference in their entirety for all purposes.
In current wireless networks, the decision to execute a handoff to a nearby base station is typically performed by the network, often using received strength signal indicator (RSSI) information provided by the mobile devices to the base stations. The information provided from a mobile device typically includes values indicating the strengths of signals received at the mobile from several nearby base stations. The communication network then makes a handoff decision according to a hysteresis rule. If the signal power from a nearby base station exceeds by a predetermined hysteresis level that of the base station currently providing the link, then a handoff to the nearby base station is executed. This technique, however, is problematic due to variations in the signal power level that are not correlated to distance from the base station. Because the signal from the mobile device scatters from nearby objects, the signal power experiences variations as the mobile device moves over small spatial scales (i.e., distances on the order of a wavelength). In addition, there are signal power variations as the mobile device moves over larger spatial scales (i.e., distances on the order of building sizes). These variations are caused by shadowing of the signal by local obstructions such as buildings, hills, and the like. In order to mitigate the effect of small scale variations, conventional handoff techniques compute an average of signal samples collected as the mobile device moves over a few wavelengths, and then applies the hysteresis rule to this average signal. This technique, however, only mitigates small scale variations. The average signal still experiences considerable fluctuation due to shadow fading on larger scales. As a result, this local shadow fading often causes inappropriate and unnecessary handoffs that deteriorate the wireless system performance and degrade the quality of service to the mobile device.
SUMMARY OF THE INVENTION
The present inventors have discovered a novel technique for significantly reducing unnecessary and inappropriate handoffs in cellular communication networks, thereby improving service and system performance. Because shadow fading is repeatable due to its strong correlation to the spatial location of a mobile device in its environment, pattern recognition techniques can be used to identify shadow fading and prevent unwanted handoffs that would otherwise have been executed using conventional techniques. More specifically, the averaged signals of mobile devices that have followed the same geographical paths will have similar shadow fading properties. According to the technique of the invention, these averaged signals are stored along with information regarding handoff decisions. The signal from a mobile device that is traveling along a known path can then be compared with the stored signal to determine if a handoff is actually necessary. The inventors provide a technique to recognize these patterns in the signals from mobile devices using probabilistic neural networks, as well as a handoff method using this pattern recognition technique.
In one aspect of the invention, a method is provided for determining handoff in a wireless communication system comprising a plurality of base stations and a mobile device. The method comprises: (a) collecting at nearby base stations signal power level information representative of signal strength between the mobile device and the nearby base stations; (b) associating the power level information with predetermined power level patterns to produce an output representing measures of similarity between the power level information and the predetermined power level patterns; (c) selecting a sequence of matching patterns from the set of predetermined power level patterns, wherein each of the matching patterns has a maximal degree of similarity with a portion of the power level information; and (d) analyzing a sequence of sets of allowable base stations to determine an appropriate base station for handoff, wherein the sequence of sets of allowable base stations correspond to the sequence of matching patterns.
Preferably, the predetermined power level patterns are correlated with shadow fading in a propagation environment of the mobile device. More specifically, the signal power level information preferably comprises a sequence of average signal power level estimates corresponding to mobile device positions separated by less than a correlation length of a log-normal shadow fading in a propagation environment of the mobile device. Each of the average signal power level estimates is preferably an average of signal power level samples corresponding to mobile device positions separated by less than a wavelength of a carrier signal of the communication system. The signal power level samples are preferably measured at times selected such that the samples correspond to mobile device positions separated by approximately equal spatial intervals.
The step of associating the power level information with the predetermined power level patterns preferably comprises applying the power level information to a probabilistic neural network whose weights are equal to training vectors derived from training power level information. Other types of neural networks or learning machines, however, may also be used. The predetermined power level patterns preferably have a predetermined sequence corresponding to a sequence of patterns observed in a training session. The step of analyzing the sequence of sets of allowable base stations to determine the appropriate base station for handoff preferably comprises selecting a base station that is an allowable base station for a subsequent pattern in a predetermined sequence of power level patterns. In a preferred embodiment, the method also comprises determining a degree to which adjacent patterns in the sequence of matching patterns meet an adjacent pattern matching tolerance value.
REFERENCES:
patent: 5175867 (1992-12-01), Wejke et al.
patent: 5570349 (1996-10-01), Bustamante et al.
patent: 5657487 (1997-08-01), Doner
patent: 5754945 (1998-05-01), Lin et al.
patent: 5884187 (1999-03-01), Ziv et al.
patent: 5901354 (1999-05-01), Menich et al.
patent: 5920549 (1999-07-01), Bruckert et al.
Cox Donald C.
Narasimhan Ravi
Hunter Daniel
Le Lana
Lumen IPS, Inc
The Board of Trustees of The Leland Stamford Junior University
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