Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1999-11-08
2003-12-16
Kizou, Hassan (Department: 2662)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C370S280000, C370S324000, C370S350000
Reexamination Certificate
active
06665288
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to communications systems and methods, and more particularly, to code division multiple access (CDMA) communications systems and methods of operation thereof.
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 (Advanced Mobile Phone Service), ETACS (Extended Total Access Communications System), NMT (Nordic Mobile Telephony)-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 (Global System for Mobile communications), 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 control channels and one or more traffic channels. A control channel is a channel used for transmitting cell identification, paging and other control information. 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 for a cell
24
. In this capacity, a base station
26
functions as a relay station for data and voice signals.
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
.
Several types of access techniques are conventionally used to provide wireless services to users of wireless systems such as those illustrated in
FIGS. 1 and 2
. Traditional analog cellular systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels, while more modern digital wireless systems may use multiple access techniques such as time division multiple access (TDMA) and/or code division multiple access (CDMA) to provide increased spectral efficiency.
CDMA systems, such as those conforming to the IS-95 standard, achieve increased channel capacity by using “spread spectrum” techniques wherein a channel is defined by modulating a data-modulated carrier signal by a unique spreading sequence, i.e., a sequence that spreads an original data-modulated carrier over a wide portion of the frequency spectrum in which the communications system operates. Conventional spread-spectrum CDMA communications systems commonly use “direct sequence” (DS) spread spectrum modulation. In direct sequence modulation, a data-modulated carrier is directly modulated by a spreading code or sequence before being amplified by a power amplifier and transmitted over a communications medium, e.g., an air interface. The spreading code typically includes a sequence of “chips” occurring at a chip rate that typically is much higher than the bit rate of the data being transmitted.
A direct sequence spread spectrum receiver typically includes a local sequence generator that locally produces a replica of a spreading sequence. This locally generated sequence is used to recover information from a transmitted spread spectrum signal that is modulated according to the same spreading sequence. Before information in a transmitted signal can be recovered, however, the locally generated spreading sequence typically must be synchronized with the spreading sequence that modulates the transmitted signal.
Synchronization of terminals is commonly achieved by transmitting a synchronization signal in each cell that a terminal can acquire to obtain a timing reference for synchronizing its de-spreading operations. For example, in an IS-95 compliant system, a “pilot channel” including a fixed carrier modulated by a known sequence is transmitted in each cell of the system, with a respective timing offset applied in a respective cell. In wideband CDMA systems currently under development, such as in WCDMA systems proposed under the 3
rd
Generation Partnership Project (3GPP), as described in Technical Specification TS 25.213, v2.3.0 (1999-9), available at http://www.3gpp.org, a downlink synchronization channel (SCH) is used to transmit a synchronization (or “search”) code at known times to provide synchronization. In the aforementioned WCDMA system, a primary synchronization code (PSC) is transmitted on a primary SCH once every slot, with the same PSC being transmitted in each cell of the system. The PSC can be detected by a terminal and used to aid the terminal in determining slot timing, as described, for example, in “Performance and Complexity of Techniques for Achieving Fast Sector Identification in an Asynchronous CDMA System,” by Ostberg et al., published in Proceedings of the 1998 Wireless Multimedia Conference, Japan, November 1998. A respective secondary synchronization code (SSC) is transmitted by a respective cell in parallel with the PSC on a secondary SCH, and identifies which of group of scrambling codes is used by the cell.
These synchronization codes are typically non-orthogonal with respect to the other modulation codes used in the system. Although this can periodically destroys orthogonality among signals and can lead to interference with other channels, it is conventionally assumed that channel coding and bit interleaving can mitigate the effects of such interference.
SUMMARY OF THE INVENTION
The present invention arises from
Ottosson Tony
Wang Yi-Pin Eric
Ericsson Inc.
Kizou Hassan
Levitan D
Myers Bigel & Sibley & Sajovec
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