Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-05-13
2001-07-24
Hunter, Daniel (Department: 2684)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S440000, C455S067700
Reexamination Certificate
active
06266529
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates in general to an improved wireless communication system and in particular, the present invention relates to an improved CDMA cellular telephone system. Still more particularly, the present invention relates to a method which allows improved sector handoff performance in highly sectorized CDMA cells in the vicinity of a base station antenna.
2. Description of the Related Art
Mobile radio telephones and mobile telephone systems are well known in the prior art. A mobile telephone system generally includes a mobile hand held radio telephone transceiver, and a base station connected to a local telephone switching system by a landline. Typically, a cellular network has an assigned set of landline telephone numbers that allows users of a mobile hand held transceiver to place and receive calls within a limited range of the base station's antenna.
Mobile hand held transceivers which are specifically designed for telephone communication are often called cellular telephones or mobile radio telephones. Cellular telephone systems have developed rapidly since the early 1980s. Persons equipped with small mobile communication devices, such as mobile radio telephone, can utilize a cellular radio system to communicate in the same way as a hard wired household telephone which utilizes landline carriers.
Due to the increase in cellular telephone utilization, digital communication is gaining popularity over analog communication. Digital communication topologies can simultaneously support many more users than analog topologies in a given frequency spectrum. Since a limited number of frequencies and channels are available, analog systems can only support a very limited number of simultaneous users. A digital radio system can handle more than 20 times the capacity of a traditional analog radio system in the same frequency spectrum. Digital systems employ methods where multiple users share the same frequency. This concept is commonly referred to as “spread spectrum communication.” Distinct digital channel sharing topologies have emerged, such as code division multiple access (CDMA), global system for mobile communication (GSM) and time division multiple access (TDMA). A digital system has a considerably larger data transmission capacity than an analog system and higher capacity translates to higher revenues for cellular system owners.
Typically, a mobile radio telephone system assigns a fixed base transceiver to geographic areas. In a typical cellular system, a geographical area is divided into small areas, called cells. Coverage is typically measured as a radius from the base station antenna. Each cell has a predefined coverage radius, for example, large cells commonly referred to as “mega cells” can have a coverage of over 20 kilometers (13 miles). Additionally, macro cells have a coverage from 1 to 20 kilometers, micro cells have a coverage of approximately 1 kilometer, while pico cells have a coverage of only 100 meters. Each cell has its own radio transceiver commonly referred to as a base station. If necessary, each cell can be further subdivided into smaller cells through cell splitting and/or sectorization by steering antenna patterns.
In a typical CDMA system, a honeycomb type pattern of cells is created which utilizes the same range of radio frequencies. In many respects, CDMA is superior to TDMA and Frequency Division Multiple Access (FDMA) because CDMA systems can utilize precisely the same frequency spectrum in all sectors without significant interference among sectors. In CDMA the same set of frequencies can also be utilized from cell site to cell site. A CDMA topology assigns a different binary sequence or code to a transmitted signal to identify the message for an individual mobile transceiver. This allows a single frequency to serve multiple users.
The specifications for CDMA operation are outline in the Electronic Industries Association/Telecommunications Industry Association (TIA/EIA) IS-95-A & TSB74 standards document entitled Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System or
CDMA Principles of Spread Spectrum Communication,
by Andrew J. Viterbi.
The correlator, a subcircuit within the mobile transceiver, accepts only energy from identified binary sequences or codes and de-spreads across the spectrum. The mobile receiver correlates its input with the desired carrier and receives the appropriate data. Received signals having codes which do not match the receiver codes are not de-spread in bandwidth and contribute only to noise. The signal to noise ratio of the desired signal is enhanced at the detector of the mobile transceiver by a factor known as the processing gain. One advantage to a CDMA system is that the receiver is not sensitive to worst case interference, but to the average interference.
CDMA has often been dismissed as unworkable in the mobile radio environment because of signal strength differential, as some users are located near the base station and others are located far away. To accommodate the far away users, a spreading bandwidth must be thousands of times greater than the data rate, making the spectral efficiency intolerable. If a reasonable bandwidth is chosen the signal cannot be received from distant users because the users near the base station significantly interfere. To overcome this inefficiency, the transmitter of each mobile is controlled such that the received powers from all users is roughly equal to achieve an interference averaging concept of power control. A similar process is performed on signals sent to the mobles from the base station(s).
Computerized switching is essential to the operation of cellular radio communication. When a communicating mobile transceiver is switched from one cell to another, a transfer of channels must take place without interruption, or at most a brief delay. The growth of electronic switching systems and the development of microprocessors have made seamless communication possible within areas covered by cellular providers. The U.S. Federal Communications Commission (FCC) continues to allocate and license additional radio frequencies. Due to increasing popularity of cellular telephones, in recent years the FCC has awarded additional frequency bands to be utilized by cellular telephone technology.
A cellular telephone system typically includes cellular subscriber units, which are portable, and cellular base stations, which are connected to the public telephone company via one or more cellular switching networks. Each cellular subscriber has an assigned cellular telephone number that allows the user to place and receive calls within a widespread range of the cellular base stations, such as throughout a metropolitan area.
Cellular telephone systems are thus based on a structure of associated cells. Cells are specified geographic areas that are defined for a specific mobile communication system where each cell has its own base station(s) and controllers) interconnected with a public telephone network.
Communication between base stations and mobile subscribers is established by negotiation protocols upon call origination. As a user passes from cell to cell, the cellular service allows calls in progress to be handed over without interruption (soft handoff) or minimal interruption (hard handoff) to adjacent cells thus providing seamless communication.
Handoffs of a mobile radio telephone between cells and sectors in CDMA ideally occur as soft handoffs. In a CDMA system, mobile radio telephone stores a list of active channels (utilized for demodulation purposes) which are being received at acceptable levels in an “active set”. The active set members are sector and/or cell channels that transmit and receive identical information with the mobile. A soft handoff occurs when the active set contains more than one sector and/or cell. When a communication path becomes weak, the mobile radio telephone will remove the weak channel from the active list, via protocols defined in IS-95, but there is n
Bracewell & Patterson L.L.P.
Chow C.
Crane John D.
Hunter Daniel
Nortel Networks Limited
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