Time difference of arrival measurement system

Communications: directive radio wave systems and devices (e.g. – Directive – Beacon or receiver

Reexamination Certificate

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Details

C455S456500

Reexamination Certificate

active

06201499

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to communication systems and more particularly to methods and apparatus for measuring the time difference of arrival for signals received at two or more receiving stations.
BACKGROUND
In conventional cellular telephone systems, a network of overlapping cells (base stations) provides a seamless two-way wireless radio communication link between a cellular phone (transmitter) and the cellular system. Each cellular phone includes a radio frequency (RF) receiver for receiving forward channel communications from a base station and a transmitter for transmitting reverse channel communications to the base station. The forward and the reverse channel communications use separate frequency bands so that simultaneous transmissions in both directions are possible.
Each base station includes a receiver for receiving reverse channel communications from a plurality of cellular transmitters and a transmitter for transmitting forward channel communications to a plurality of cellular transmitters. Adjacent cells are assigned different frequency channels to avoid interference. The number of frequency channels available for communications is limited. Time division multiple access (TDMA) or frequency division multiple access (FDMA) communication techniques may be employed to increase the number of simultaneous calls handled by a cellular system.
In conventional cellular systems, base station controllers (BSC) monitor and control one or more base stations. The number of base stations is typically between several tens to several hundreds. A base station controller performs frequency administration, control functions for each base station and exchange functions. Base station controllers assign RF carriers (channels) to support calls, coordinate the handoff of cellular communications between base stations and monitor and report on the status of each base station under its control. Base station controllers may be located at the same site as a base station or remote from it. Base station controllers and the base stations which they support form a functional unit referred to as the base station subsystem (BSS).
A mobile services switching center (MSC) provides the interface between the cellular system and the public switched telephone network (PSTN). The MSC provides switching exchange services for routing calls from the fixed PSTN network through the BSC and base stations to individual cellular transmitters.
An operation and maintenance center (OMC) controls traffic load and error messaging for the BSCs and the base stations and interfaces both with the MSC switches and the BSCs.
Analog cellular telephones in the United States generally adhere to the Advance Mobile Phone Service (AMPS) protocol as defined in the EIA/TIA 553 specification. The cellular telephone infrastructure coordinates the transmissions of the mobile cellular telephones. Cellular base stations send commands to the cellular telephones directing them to transmit at certain times and at certain frequencies. When a base station initiates communication with a cellular telephone, it sends the cellular telephone a mobile station control message with a page order on its forward control channel. The cellular telephone sends a page response message on the reverse control channel. After a call is setup and the cellular telephone and the cellular base station are communicating on the reverse and forward voice channels respectively, the cellular base station may send a mobile station control message to the telephone on the forward voice channel to order the telephone to a new frequency, change its power, or command it to respond to verify it is still communicating.
An analog cellular telephone conforming to the AMPS protocol forms a baseband signal consisting of an analog voice signal in the 300 to 3000 Hz frequency range and a supervisory audio tone (SAT) at 5970, 6000, or 6030 Hz. The baseband signal is frequency modulated to form an RF signal. The instantaneous frequency deviation due to voice modulation is less than +/−12 kHz. The instantaneous frequency deviation due to SAT modulation is typically +/−2 kHz. The SAT tone is always present. The voice signal comes and goes depending upon how loud the user of the cellular telephone is speaking.
Digital cellular telephones transmit voice information in a digital format rather than in an analog format. For a given call, a reverse digital traffic channel (RDTC) carrier frequency and slot designation to be used are received by the digital cellular telephone in a message from the base station. At call setup, this message is received on a forward control channel, and for a handoff, the forward traffic channel is used. The RDTC has a center frequency, slot timing, carrier modulation, data format and communications protocol as defined by the TIA/EIA IS-54 or IS-136 specifications (hereafter referred to generically as IS-136).
During an in-progress call, a digital cellular telephone transmits voice data to the appropriate base station using the RDTC. The digital cellular telephone does not transmit continuously on its RDTC, but instead transmits in bursts which are adjusted in time to arrive at the base station during a particular time slot. The use of non-overlapping receive time slots at the base station allows more than one mobile station to have simultaneous in-progress calls to a given base station on a given carrier frequency, using a time-division multiple access (TDMA) technique. More than one transmitter can use the same carrier frequency and time slot, but it is intended that the path loss between any two such transmitters is large enough not to cause unacceptable interference at the desired receiver.
Locating Cellular Telephones
Cellular telephones provide unique challenges for security and privacy considerations. Because the cellular transmissions are broadcast out over the open airways, cellular transmissions may be intercepted for unscrupulous purposes. Cellular telephones may be stolen or their identifiers (MIN/ESN identification codes) misappropriated. In response to these security concerns, cellular providers have sought to combat improper use by providing cellular telephone location systems for use in conjunction with a cellular communication system.
Typically, a cellular telephone location system makes use of triangulation or trilateration (hyperbolic) techniques to locate a selected cellular transmitter. The position of a cellular transmitter can be determined using time difference of arrival (TDOA) information based on signal arrival-time measurements for a transmitted signal received at three or more receiving sites. In a system employing a trilateration technique, multiple receiving sensors (three or more) measure the time of arrival of the same transmitter signal. The time of arrival measurements at the receiving sensors may be transmitted to a central processor where the time difference of arrival data is calculated and used to perform an estimation of the location and velocity of the transmitter.
The estimation typically includes forming hyperbolas from the TDOA calculations between the three or more receiving sensors. Transmitter location is estimated from the intersection of two or more independently generated hyperbolas determined from three or more receiving sensors. Methods for determining RF transmitter location based on time difference of arrival are discussed in greater detail in “Statistical Theory of Passive Location Systems” by Don J. Torrieri (IEEE Transactions on Aerospace and Electronic Systems, Vol. AE, 5-20, No. 2, March 1984, pp. 183-198) which is expressly incorporated herein by reference.
In addition to locating illegal phones, location systems may be used to locate callers placing calls into the emergency telephone system (911) or callers seeking roadside assistance through the cellular communication system, for vehicle or package tracking, for cellular phone billing based on location and for various other law enforcement purposes. Recognizing the importance of location s

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