Multiplex communications – Communication over free space – Combining or distributing information via time channels
Reexamination Certificate
1998-06-22
2002-10-01
Nguyen, Chau (Department: 2663)
Multiplex communications
Communication over free space
Combining or distributing information via time channels
C370S506000, C375S358000
Reexamination Certificate
active
06459696
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates, in general, to a method and system, to be utilized with wireless communications systems, having cellular architectures which utilize digital clocked systems (such as Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), or similar technologies), and which interface with public switched telephone networks (PSTNs). In particular, the present invention relates to a method and system, to be utilized with wireless communications systems, having cellular architectures which utilize digital clocked systems (such as TDMA, CDMA, or similar technologies), and which interface with PSTNs, wherein the method and system increase the reliability of such wireless communications systems by avoiding communication failures at the wireless communication system-PSTN system interfaces.
2. Description of the Related Art
The present invention is related to wireless communication systems, and, in particular, to wireless communications systems which have both a cellular architecture (e.g., cellular telephony, personal communications systems) and which utilize CDMA (or similar technologies) and which interface with public switched telephone networks (PSTNs). Wireless communication refers to the fact that transmission between sending and receiving stations occurs via electromagnetic radiation (e.g., microwave) not guided by any hard physical path. Cellular architecture refers to the fact that the wireless system effects service over an area by utilizing a system that can (ideally) be pictographically represented as a cellular grid. CDMA stands for Code Division Multiple Access, which is a type of spread spectrum technology, originally developed for military application and thereafter adapted for civilian use.
Wireless cellular communication utilizing CDMA is the latest incarnation of a technology that was originally known as mobile telephone systems. Early mobile telephone system architecture was structured similar to television broadcasting. That is, one very powerful transmitter located at the highest spot in an area would broadcast in a very large radius. If a user were in the usable radius, then that user could broadcast to the base station and communicate by radio telephone to the base station. However, such systems proved to be very expensive for the users and not very profitable to the communication companies supplying such services. The primary limiting factor, or problem, of the original mobile telephone systems was that the number of channels available for use was limited due to severe channel-to-channel interference within the area served by the powerful transmitter.
This problem was solved by the invention of the wireless cellular architecture concept. The wireless cellular architecture concept utilizes geographical subunits called “cells” and encompasses what are known as the “frequency reuse” and “handoff” concepts. A cell is the basic geographic unit of a cellular system. Cells are defined by base stations (a base station consists of hardware located at the defining location of a cell and includes power sources, interface equipment, radio frequency transmitters and receivers, and antenna systems) transmitting over small geographic areas that are represented (ideally) as hexagons. The term “cellular” comes from the honeycomb shape of the areas into which a coverage region, served via two or more base stations, is divided when the mathematically ideal hexagonal shape is used to represent the usable geographic area of each of the two or more base stations. It is to be understood that, although the mathematically ideal shape of the cell is a hexagon, in practicality each cell size varies dependent upon the landscape (e.g., a base station transmitting on a flat plane will closely approximate the ideal hexagon, whereas a base station transmitting in a valley surrounded by hills will not closely approximate a hexagon due to the interference from the surrounding hills).
The first large-scale wireless communications system utilizing cellular architecture in North America was the Advanced Mobile Phone Service (AMPS) which was released in 1983. With the introduction of AMPS, user demand for bandwidth was initially low until users became acquainted with the power of the system. However, once users became acquainted with the power of cellular, the demand for the service increased. Very quickly, even the extended number of channels available utilizing the cellular concepts of reduced power output and frequency reuse were quickly consumed by user demand in certain geographic areas, and a problem arose with respect to capacity.
Engineers responded to the problem by devising the Narrowband Analog Mobile Phone Service (NAMPS). NAMPS utilizes frequency division multiplexing to transmit three transmit/receive channels in the same bandwidth wherein AMPS had previously only transmitted one transmit/receive channel. Thus, NAMPS essentially tripled the capacity of AMPS. However, even though NAMPS essentially tripled the capacity of AMPS, the extended number of channels available with NAMPS were quickly consumed by user demand in certain geographic areas, and a problem again arose with respect to capacity.
Engineers responded to this new problem by devising Digital AMPS (or DAMPS, also known as TDMA). In DAMPS/TDMA time division multiple access techniques are utilized to multiplex user data together. Furthermore, digital data compression techniques are utilized at both the transmission and reception ends. These techniques give rise to increased capacity, and clarity, even exceeding that of NAMPS. However, as was the case with both AMPS and NAMPS, the increased bandwidth capacity of DAMPS/TDMA has been quickly consumed by user demand in certain geographic areas.
Subsequent attempts to increase cellular telephony bandwidth capacity tended to be variations on the foregoing described themes. However, it became apparent that some new communications technology would be necessary to give rise to any significant increase in bandwidth beyond that available with the foregoing described technologies. It was decided within the industry that such new technology would be standard CDMA, which stands for Code Division Multiple Access.
Notice that in all the foregoing described technologies, the method of using multiple transmit/receive channels with each such transmit/receive channel utilizing a different pair of frequencies was maintained throughout. Standard CDMA breaks completely with this method of communication.
Standard CDMA utilizes cellular architecture and a type of hand-off. However, in standard CDMA, transmission and reception is done by all users on the same frequency. Standard CDMA is able to achieve this feat by insuring that the signals from different users are adjusted such that the signals do not interfere with each other to the point of being unable to understand the messages from the different users.
The way in which standard CDMA works is somewhat analogous to a situation in which two English speaking persons are communicating in a room wherein many other non-English speakers are also communicating in a language which the two English speakers do not understand. Since the two English speakers do not understand the language spoken by the non-English speakers in the room, the conversations of their non-English-speaking counterparts will be interpreted by the two English speakers as meaningless “noise.” Consequently, since the English speakers will attach no meaning to the “noise,” the English speakers will be able to disregard the “noise” and continue to engage in their conversation provided that they both speak loudly enough so that each can be understood by the other despite the “noise” generated by their non-English-speaking counterparts. This is true even though all persons in the room are talking, or communicating, in the same band of sound frequencies which the human ear can hear.
Standard CDMA is able to achieve the same affect by modulating the signal of each user within a particular cell with a “pseudo-no
Carpenter Frank
Jackson Patrick
Lauson Dave
Wong Henry
Bracewell & Patterson L.L.P.
Crane John D.
Nguyen Chau
Nortel Networks Limited
Trinh D.
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