Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1999-03-17
2002-11-12
Chin, Stephen (Department: 2664)
Pulse or digital communications
Receivers
Interference or noise reduction
C375S354000, C375S366000
Reexamination Certificate
active
06480558
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to wireless communications systems and methods, and more particularly, to apparatus and methods for synchronization and cell search in wireless communications systems.
BACKGROUND OF THE INVENTION
Wireless communications systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990's. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in
The Mobile Communications Handbook,
edited by Gibson and published by CRC Press (1996).
FIG. 1
illustrates a typical terrestrial cellular radiotelephone communication system
20
. The cellular radiotelephone system
20
may include one or more radiotelephones (terminals)
22
, communicating 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 network may include hundreds of cells, may include more than one MTSO, and may serve thousands of radiotelephones.
The cells
24
generally serve as nodes in the communication system
20
, from which links are established between radiotelephones
22
and the MTSO
28
, by way of the base stations
26
serving the cells
24
. Each cell
24
will have allocated to it one or more dedicated control channels and one or more traffic channels. A 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 may be effected between two mobile terminals
22
or between a mobile terminal
22
and a landline telephone user
32
through a public switched telephone network (PSTN)
34
. The function of a base station
26
is to handle radio communication between a cell
24
and mobile terminals
22
. In this capacity, a base station
26
functions as a relay station for data and voice signals.
Those skilled in the art will appreciate that “cells” may have configurations other than the omnidirectional cells
24
illustrated in FIG.
1
. For example, the coverage areas conceptually illustrated as a hexagonally-shaped area served by a base station
26
may actually be subdivided into three sectors using separate directional antennas mounted at the base station
26
, with the sector antenna having patterns extending in three different directions. Each of these sectors may in itself be considered a “cell.” As will be appreciated by those skilled in the art, other cell configurations are also possible, including, for example, overlaid cells, microcells, picocells and the like.
As illustrated in
FIG. 2
, a satellite
42
may be employed to perform similar functions to those performed by a conventional terrestrial base station, for example, to serve areas in which population is sparsely distributed or which have rugged topography that tends to make conventional landline telephone or terrestrial cellular telephone infrastructure technically or economically impractical. A satellite radiotelephone system
40
typically includes one or more satellites
42
that serve as relays or transponders between one or more earth stations
44
and terminals
23
. The satellite conveys radiotelephone communications over duplex links
46
to terminals
23
and an earth station
44
. The earth station
44
may in turn be connected to a public switched telephone network
34
, allowing communications between satellite radiotelephones, and communications between satellite radio telephones and conventional terrestrial cellular radiotelephones or landline telephones. The satellite radiotelephone system
40
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
48
, each serving distinct geographical coverage areas
50
in the system's service region. The coverage areas
50
serve a similar function to the cells
24
of the terrestrial cellular system
20
of FIG.
1
.
Traditional analog cellular 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. In a typical FDMA system, each of these discrete frequency bands serves as a channel over which cellular radiotelephones communicate with a cell, through the base station or satellite serving the cell.
The limitations on the available frequency spectrum present 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 channels 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 having only several hundred allocated frequency bands.
Another technique which can further increase channel capacity and spectral efficiency is the use of 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. Communications over a frequency band typically occur on a repetitive TDMA frame structure that includes a plurality of time slots. Examples of systems employing TDMA are those conforming to the dual analog/digital IS-54B standard employed in the United States, in which each of the frequency bands of the traditional analog cellular spectrum are subdivided into 3 time slots, and systems conforming to the GSM standard, which divides each of a plurality of 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.
Yet another technique for potentially increasing system capacity is to employ “spread spectrum” code division multiple access (CDMA) techniques. In a system employing spread spectrum techniques, a channel may be defined by modulating a data-modulated carrier signal by a unique spreading code, i.e., a code that spreads an original data-modulated carrier over a wide portion of the frequency spectrum in which the communications system operates. Data may be recovered from the transmitted signal by demodulating the signal u
Ottosson Tony
Wang Yi-Pin Eric
Chin Stephen
Ericsson Inc.
Kim Kevin
Myers Bigel & Sibley & Sajovec
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