Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-08-11
2003-02-25
Urban, Edward F. (Department: 2685)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S443000, C455S450000, C455S453000
Reexamination Certificate
active
06526279
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to communication systems and, more particularly, to mobile terminals operating with two or more wireless communications networks.
BACKGROUND OF THE INVENTION
Public cellular networks (public land mobile networks) are commonly employed to provide voice and data communications to a plurality of subscribers. For example, analog cellular radiotelephone systems, such as designated AMPS, ETACS, NMT450, and NMT-900, have been deployed successfully throughout the world. More recently, digital cellular radiotelephone systems such as that designated as IS-54B (and its successor IS-136) in North America and the pan-European GSM system have been introduced. These systems, and others, are described, for example, in the book titled
Cellular Radio Systems
by Balston, et al., published by Artech House, Norwood, Mass., 1993. In addition, satellite based radio communication systems are also being utilized to provide wireless communications in various regions such as the Asian Cellular Satellite System (ACeS) generated by Lockheed Martin Corporation. Furthermore, dual-mode mobile terminals are known which allow a single terminal to access to different networks. For example, an analog/digital dual-mode terminal or a terrestrial/satellite dual-mode terminal may be desirable in various geographic areas to maximize the communications capabilities available to a user.
FIG. 1
illustrates a conventional terrestrial wireless communication system
20
that may implement one of the aforementioned wireless communication standards. The wireless system may include one or more wireless mobile terminals
22
that communicate with a plurality of cells
24
served by base stations
26
and a mobile telephone switching office (MTSO)
28
. Although only three cells
24
are shown in
FIG. 1
, a typical cellular radiotelephone network may comprise hundreds of cells, and may include more than one MTSO
28
and may serve thousands of wireless mobile terminals
22
.
The cells
24
generally serve as nodes in the communication system
20
, from which links are established between wireless mobile terminals
22
and a MTSO
28
, by way of the base stations
26
servicing the cells
24
. Each cell
24
will have allocated to it one or more dedicated control channels and one or more traffic channels. The control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the communication system
20
, a duplex radio communication link
30
may be effected between two wireless mobile terminals
22
or between a wireless mobile terminal
22
and a landline telephone user
32
via a public switched telephone network (PSTN)
34
. The function of the base station
26
is commonly to handle the radio communications between the cell
24
and the wireless mobile terminal
22
. In this capacity, the base station
26
functions chiefly as a relay station for data and voice signals.
FIG. 2
illustrates a conventional celestial wireless communication system
40
. The celestial wireless communication system
40
may be employed to perform similar functions to those performed by the conventional terrestrial wireless communication system
20
of FIG.
1
. In particular, the celestial wireless communication system
40
typically includes one or more satellites
42
that serve as relays or transponders between one or more earth stations
44
and satellite wireless mobile terminals
23
. The satellite
42
communicates with the satellite wireless mobile terminals
23
and earth stations
44
via duplex communication links
46
. Each earth station
44
may, in turn, be connected to a PSTN
34
, allowing communications between the wireless mobile terminals
23
and conventional landline telephones
32
(FIG.
1
).
The celestial wireless communication system
40
may utilize a single antenna beam covering the entire area served by the system, or as shown in
FIG. 2
, the celestial wireless communication system
40
may be designed such that it produces multiple, minimally-overlapping beams
48
, each serving a distinct geographical coverage area
50
within the system's service region. A satellite
42
and coverage area
50
serve a function similar to that of a base station
26
and cell
24
, respectively, of the terrestrial wireless communication system
20
.
Thus, the celestial wireless communication system
40
may be employed to perform similar functions to those performed by conventional terrestrial wireless communication systems. In particular, a celestial radiotelephone communication system
40
has particular application in areas where the population is sparsely distributed over a large geographic area or where rugged topography tends to make conventional landline telephone or terrestrial wireless infrastructure technically or economically impractical.
Traditional analog radiotelephone systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels. As a practical matter well-known to those skilled in the art, radiotelephone communications signals, being modulated waveforms, typically are communicated over predetermined frequency bands in a spectrum of carrier frequencies. These discrete frequency bands serve as channels over which cellular radiotelephones communicate with a cell, through the base station or satellite serving the cell. In the United States, for example, Federal authorities have allocated to cellular communications a block of the UHF frequency spectrum further subdivided into pairs of narrow frequency bands, a system designated EIA-553 or IS-19B. Channel pairing results from the frequency duplex arrangement wherein the transmit and receive frequencies in each pair are offset by 45 Mhz.
A defined range of radio channels are allocated to cellular mobile communications in the United States. The limitations on the number of available frequency bands present several challenges as the number of subscribers increases. Increasing the number of subscribers in a cellular radiotelephone system generally requires more efficient utilization of the limited available frequency spectrum in order to provide more total channels while maintaining communications quality. This challenge is heightened because subscribers may not be uniformly distributed among cells in the system. More channels may be needed for particular cells to handle potentially higher local subscriber densities at any given time. For example, a cell in an urban area might conceivably contain hundreds or thousands of subscribers at any one time, easily exhausting the number of frequency bands available in the cell.
For these reasons, conventional cellular systems employ frequency reuse to increase potential channel capacity in each cell and increase spectral efficiency. Fixed frequency reuse involves allocating frequency bands to each cell, with cells employing the same frequencies geographically separated to allow radiotelephones in different cells to simultaneously use the same frequency without interfering with each other. By so doing, many thousands of subscribers may be served by a system of only several hundred frequency bands.
An alternative approach to fixed frequency reuse (with or without frequency hopping) is adaptive channel allocation (ACA). In ACA networks, the available channels are typically dynamically allocated throughout the network to maximize system capacity rather than defining a specific subset of the available channels for each cell within the network. The allocation may be based on measurements made by the mobile of channels (or frequencies) which are potential sources of interference signals as contrasted with the selection of candidate channels for handoff as provided with mobile assisted handoff in some fixed frequency reuse networks. These measurements are made to determine the level of interference signals on the various channels. The interference signal measurements may, in turn, be used to select a channel which may provi
Ericsson Inc.
Gesesse Tilahun
Myers Bigel & Sibley & Sajovec
Urban Edward F.
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