Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2000-08-09
2003-06-17
Vo, Nguyen T. (Department: 2682)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S450000
Reexamination Certificate
active
06580909
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of mobile wireless data communications and, more particularly, to a communications system where each communications unit is capable of determining its own geographic location e.g., from the Global Positioning System (GPS) of satellites, and forming with other units a stand-alone baseless network.
BACKGROUND OF THE INVENTION
There are many types of systems where a plurality of mobile units geographically dispersed must all communicate together in a distributed, decentralized form, using a two-way flow of data over a single communications channel. Examples of mobile wireless systems are aircraft location and collision avoidance systems, telemetry and proximity radar systems, dynamic navigation, surveillance and emergency location systems, air combat monitoring systems, traffic alert systems, pilot data systems, multiple vehicle tracking and security systems, fleet control systems, mobile work force systems or location based adaptive radio control systems. In each case no static master-slave relationship exists among those mobile units. On the contrary, they are all functionally equivalent and, generally, the information coming from any one unit is made available and transmitted to all others at some point of time. Because this occurs on a common shared communications channel, as the number of units that try to access this common channel increases and they are more and more used, the time left to each of them may dramatically be impacted up to a point where messages transmitted by several units collide. Beyond this point probability of transmission collisions dramatically increases since units experiencing collisions all try to retransmit. This may reach a point where no communication becomes possible. Therefore, in such systems, it is of the utmost importance that the common communications channel implement some kind of “distributed” or “universal” channel coordination or control mechanism aimed at preventing transmissions collision while it remains equally important that the common communications channel be used as efficiently as possible so as it can be shared between many users.
Also, a wireless communications environment is characterized by high error rates and by large variations on the signal propagation conditions over time and position. Parameters like distance, weather, radio shadow and multi-path propagation may dramatically affect the transmission conditions. Increasing the power of the transmitters to overcome this is limited in practice, because is mobile units means are generally portable units with limited battery capacity and safety regulations limiting maximum allowed Radio Frequency (RF) signals strength from transmitters. Thus, there is a maximum range for reliable reception of the information transmitted by distant units which practically limits the possibility that the information transmitted from any one unit can correctly be received directly by all the others. Therefore, in such systems, it is also of the utmost importance that each one of the mobile units, receiving a signal from any other unit, can also play the role of a regenerator or relay, thus eventually allowing to deliver reliable information originated from a set of units to other sets that cannot otherwise communicate directly.
Actually, methods have been proposed for enabling data communications over a fleet of mobile units. For example, U.S. Pat. No. 5,153,836, entitled “Universal Dynamic Navigation, Surveillance, Emergency Location, and Collision Avoidance System and Method,” teaches how each craft must broadcast its position, identifying information and other messages on a regular basis, without having to be solicited, while preventing conflicts with any radio frequency signals possibly present in the airspace. Similarly, U.S. Pat. No. 4,835,537, entitled “Telemetry Burst Collision Avoidance System,” describes a system where each vehicle repeatedly broadcasts its current position and uses carrier detection to prevent transmitting when another station is active. Also, U.S. Pat. No. 4,197,538, entitled “Pilot's Traffic Monitoring System,” teaches how, with respect to a given aircraft, position and altitude of all other air craft are received while aircraft position and altitude is transmitted by an onboard FM transceiver multiplexer operated within a transmit-receive duty cycle and providing for transmission collisions. U.S. Pat. No. 5,907,293, entitled “System for Displaying the Characteristics, Position, Velocity and Acceleration of Nearby Vehicles on a Moving-map,” uses a GPS receiver to determine a vehicle position with respect to fixed positioned radars at blind intersections, determining other vehicle positions and broadcasting vehicle location information for vehicles lacking a means to track their own location, and acknowledges that broadcast messages could possibly collide. U.S. Pat. No. 5,872,526, entitled “GPS Collision Avoidance System,” describes how crafts equipped with GPS receivers are broadcasting their current location to other vehicles while vehicles not equipped with GPS are receiving location information from common ground equipment broadcasted to all vehicles and also acknowledges that transmission collisions occur.
A different approach for controlling transmissions in a distributed mobile environment is that of U.S. Pat. No. 5,367,524, entitled “Method for Sequential Data Transmission,” whereby using accurate GPS clock signals as an external clock, a communications system determines a cycle time and duration of time slots such that each communications unit is allocated a time slot for data transmission. This approach eliminates the need for polling the communications units and reduces substantially data congestion, but requires some Time Division Multiplex (TDM) master controller to allocate different time slots to different units. A similar communications control approach is proposed in U.S. Pat. No. 5,587,904, entitled “Air Combat Monitoring System and Methods and Apparatus Useful Therefor,” where information dissemination among a plurality of aircraft is performed by employing GPS signals for synchronizing transmitters and uses TDM for communicating among the aircrafts.
Yet another approach is that of U.S. Pat. No. 5,636,123, entitled “Traffic Alert and Collision Avoidance Coding System,” that describes a traffic alert and collision avoidance system where the airspace is divided into a grid of volume elements, each of which is assigned a unique pseudo noise (PN) code and where a vehicle determines its position by means of a GPS receiver and places it within one of the volume elements in the airspace. The message to transmit is then generated by modulating a carrier signal with the PN code representing the volume element containing the transmitting vehicle and with the navigation message being then transmitted on a common communications channel using time-multiplexing based on a pseudo random sequence; each receiving vehicle only tracks collision avoidance signals produced by vehicles located in its own and surrounding volume elements.
Still another different approach corresponds to U.S. Pat. No. 4,380,050, entitled “Aircraft Location and Collision Avoidance System,” which describes a system where the azimuth and range information of an aircraft with respect to a common ground reference is made available to other aircraft by transmission of a pulse at a time uniquely associated with the aircraft's location so that the collision of the pulses transmitted by air crafts from different locations is impossible, although the method is limited to proximity detection, not enabling transmission of other relevant information, like speed, heading or craft identification. In part based on the same principle, U.S. Pat. No. 5,450,329, entitled “Vehicle Location Method and System,” the time slot in which the data message is transmitted is uniquely assigned to each transmitting vehicle as a function of its position, determined by means of a GPS receiver or alike system, in a cartographic grid segme
International Business Machines - Corporation
Ly Nghi H.
Vo Nguyen T.
Winstead Sechrest & Minich
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