Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-07-30
2003-06-03
Chin, Stephen (Department: 2634)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S144000, C375S346000
Reexamination Certificate
active
06574270
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to communications methods and apparatus, and more particularly, to spread spectrum communications methods arid apparatus.
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 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, wherein discrete frequency bands serve as channels over which cellular terminals communicate with cellular base stations. Typically, these bands are reused in geographically separated cells in order to increase system capacity.
Modern digital wireless systems typically utilize different multiple access techniques such as time division multiple access (TDMA) and/or code division multiple access (CDMA) to provide increased spectral efficiency. In TDMA systems, such as those conforming to the GSM or IS-136 standards, carriers are divided into sequential time slots that are assigned to multiple-channels such that a plurality of channels may be multiplexed on a single carrier. 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 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.
Conventional spread-spectrum CDMA communications systems commonly use “direct sequence” 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.
In a typical CDMA system, a data stream intended for a particular user (terminal) is first direct-sequence spread according to a user-specific spreading sequence, and the resultant signal is then scrambled according to a cell-specific scrambling sequence. The spread and scrambled user data stream is then transmitted in a communications medium. Spread-spectrum signals for multiple users generally combined to form composite signal in the communications medium.
Downlink signals for different physical channels within a cell are typically transmitted from a base station in a synchronous fashion. The user-specific spreading codes are typically orthogonal, creating mutually orthogonal downlink signals at the transmitter. However, channel dispersion typically results in a loss of orthogonality at the receiver, giving rise to intra-cell multi-user interference that can lead to degradation of receiver performance. This interference can be exacerbated by the “near-far” problem, i.e., the higher contribution of signal energy from interferers that are closer to the receiver than the station transmitting a desired signal. Although the near-far problem can be alleviated by power control techniques on the uplink, power control is generally impractical on the downlink.
These problems may be exacerbated in “third generation” systems such as wideband CDMA (WCDMA) systems. Such systems typically are intended to support several kinds of communications services, including voice and data applications that have varying information rate requirements. Generally, these systems are designed to support higher data rates than their predecessors, and also are designed to support a wide variety of data rates. However, the higher data rates and increased bandwidth of such wideband systems can combine to cause severe inter-cell and intra-cell interference among users. Such wideband third-generation systems are typically designed to support multiple spreading factors, which means that signals with low spreading factors generally require higher transmit power to achieve the same link quality as signals transmitted using higher spreading factors. This power differential can further increase intra-cell
Gupta Someshwar C.
Madkour Mohamed F.
Wang Yi Pin Eric
Ericsson Inc.
Lugo David B.
Myers Bigel & Sibley & Sajovec
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