Multiplex communications – Communication over free space – Repeater
Reexamination Certificate
1996-11-19
2003-08-12
Kizou, Hassan (Department: 2662)
Multiplex communications
Communication over free space
Repeater
C370S337000, C370S522000, C455S466000
Reexamination Certificate
active
06606309
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to communications systems and methods, in particular, to radiotelephone communications systems and methods.
BACKGROUND OF THE INVENTION
Cellular radiotelephone systems 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, NMT-450, and NMT-900, have been deployed successfully throughout the world. More recently, digital cellular radiotelephone systems such as designated IS-54B 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.
FIG. 1
illustrates a typical terrestrial cellular radiotelephone communication system
20
as in the prior art. The cellular radiotelephone system may include one or more radiotelephones
21
, communicating with a plurality of cells
36
served by base stations
23
and a mobile telephone switching office (MTSO)
25
. Although only three cells
36
are shown in FIG.
1
, a typical cellular network may comprise hundreds of cells, may include more than one MTSO, and may serve thousands of radiotelephones.
The cells
36
generally serve as nodes in the communication system
20
, from which links are established between radiotelephones
21
and the MTSO
25
, by way of the base stations
23
serving the cells
36
. Each cell 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 cellular network
20
, a duplex radio communication link
32
may be effected between two mobile stations
21
or between a radiotelephone
21
and a landline telephone user
33
. The function of the base station
23
is commonly to handle the radio communication between the cell and the mobile station
21
. In this capacity, the base station
23
functions chiefly as a relay station for data and voice signals.
As illustrated in
FIG. 2
, satellites
110
may be employed to perform similar functions to those performed by base stations in a conventional terrestrial radiotelephone system, for example, in areas where population is sparsely distributed over large areas or where rugged topography tends to make conventional landline telephone or terrestrial cellular telephone infrastructure technically or economically impractical. A satellite radiotelephone system typically includes one or more satellites
110
which serve as relays or transponders between one or more earth stations
130
and radiotelephones
21
. The satellite communicates with radiotelephones
21
and earth stations
130
over duplex links
170
. The earth station may in turn be connected to a public switched telephone network
140
, allowing communications between satellite radiotelephones, and communications between satellite radio telephones and conventional terrestrial cellular radiotelephones or landline telephones. The satellite radiotelephone system may utilize a single antenna beam covering the entire area served by the system, or, as shown, the satellite may be designed such that it produces multiple minimally-overlapping beams
150
, each serving distinct geographical coverage areas
160
in the system's service region. A satellite
110
and coverage area
160
serve functions similar to that of a base station
23
and cell
36
, respectively, in a terrestrial cellular system.
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. At present there are 832, 30-Khz wide, radio channels allocated to cellular mobile communications in the United States.
The limitations on the number of available frequency bands presents several challenges as the number of subscribers increases. Increasing the number of subscribers in a cellular radiotelephone system 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. 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.
Another technique which may further increase channel capacity and spectral efficiency is time division multiple access (TDMA). A TDMA system may be implemented by subdividing the frequency bands employed in conventional FDMA systems into sequential time slots, as illustrated in FIG.
3
. Although communication on frequency bands f
1
-f
m
typically occur on a common TDMA frame
310
that includes a plurality of time slots t
1
—t
n
, as shown, communications on each frequency band may occur according to a unique TDMA frame, with time slots unique to that band. Examples of systems employing TDMA are the dual analog/digital IS-54B standard employed in the United States, in which each of the original frequency bands of EIA-553 is subdivided into 3 time slots, and the European GSM standard, which divides each of its frequency bands into 8 time slots. In these TDMA systems, each user communicates with the base station using bursts of digital data transmitted during the user's assigned time slots. A channel in a TDMA system typically includes one or more time slots on one or more frequency bands.
Because it generally would be inefficient to permanently assign TDMA time slots to a radiotelephone, typical radiotelephone systems assign time slots on an as-needed basis to more efficiently use the limited carrier frequency spectrum available to the system. Therefore, a critical task in radiotelephone communications is providing a radiotelephone with access to the system, i.e., assigning time slots corresponding to a voice or data channel to a radiotelephone when it desires to communicate with another radiotelephone or with a landline telephone or conventional cellular radiotelephone via the PSTN. This task is encountered both when a radiotelephone attempts to place a call and when a radiotelephone attempts to respond to a page from another radiotelephone or conventional telephone.
Access to a radiotelephone communications system may be provided in a number of ways. For example, a polling technique may be utilized whereby a central or base station seriall
Ericsson Inc.
Kizou Hassan
Myers Bigel & Sibley & Sajovec
Qureshi Afsar M.
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