Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-01-22
2002-12-31
Chin, Stephen (Department: 2734)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S347000
Reexamination Certificate
active
06501788
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to communications systems and methods, and more particularly, to spread spectrum communications systems and methods.
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 been 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), as described in
The Mobile Communications Handbook
, edited by Gibson and published by CRC Press (1996). Proposed next-generation systems utilizing technology such as wideband code division multiple access (W-CDMA) will offer a wide array of multimedia services.
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
. 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.
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 frequency division multiple access (FDMA) to create communications channels. However, the tremendous increase in the number of users of wireless services and the demand for data and other non-voice services have led to the development of other techniques that can utilize the available spectrum in a more efficient manner. These more advanced techniques include time division multiple access (TDMA), in which communications from multiple users are time-multiplexed on frequency bands in system-defined time “slots,” and “spread spectrum” or code division multiple access (CDMA) techniques in which channels of a system are defined by modulating data-modulated carrier signals by unique spreading codes, i.e., codes that spread data-modulated carriers over the frequency spectrum in which the communications system operates. The use of unique spreading codes for channels allows several users to effectively share the same bandwidth.
In proposed wideband CDMA systems, such as a W-CDMA system conforming to the UMTS/IMT-2000 specifications, downlink (base station to subscriber terminal) signals for different channels within a cell are transmitted synchronously by the base station using a scrambling code specific to the cell. Typically, orthogonal channelization codes or sequences, also known as spreading codes, are assigned to distinct physical channels transmitted in a cell, thus creating orthogonal downlink signals within the cell. If the communications medium in which the signals are transmitted does not introduce delay spread, this orthogonality may be maintained at the receiving terminal, thus reducing the likelihood of multi-user (inter-user) interference. However, if the communications medium in which the signals are transmitted introduces delay spread, orthogonality may not be maintained at the receiving terminal. This can increase multi-user interference, and may degrade performance.
Performance may be severely degraded in the presence of a so-called “near-far” problem, i.e., when a weak desired signal is received at a receiving station along with a strong interfering signal. In a typical uplink to a base station, this problem may be managed by power control techniques, e.g., by boosting the desired signal such that all signals arrive at the base station at substantially the same power. However, such power control typically is not feasible for a downlink to a subscriber terminal.
It is known that a signal transmitted over a wide band of frequencies generally may produce more multipath signal components than a signal transmitted over a narrower bandwidth. Thus, for example, a channel in a wideband CDMA system generally exhibits a higher degree of dispersiveness than a channel in a narrower bandwidth system such as a system conforming to the IS-95 CDMA standard. Consequently, W-CDMA systems generally have a higher likelihood of multi-user interference than their narrower-bandwidth precursors.
Moreover, proposed W-CDMA systems that allow for the use of variable spreading factors to allow users to achieve varying data rates may be more vulnerable to multi-user interference. For example, proposed W-CDMA systems envision the use of high spreading factors (on the order of 128) for voice channels and the use of lower spreading factors for high-speed data services. If such voice and data services are designed to exhibit comparable link quality, i.e., comparable end-to-end user data reliability, the low-spreading factor signals will generally be transmitted with much higher power than the high spreading factor signals. This power discrepancy may exacerbate multi-user interference in a dispersive medium.
Variable spreading factor schemes which allow for the concurrent use of high and low spreading codes can also exacerbate the “near-far” problem. For example, a user located near the edge of a cell transmitting with a low spreading factor and high power may significantly interfere at a receiving terminal with a high-spreading factor, low-power user positioned nearer the receiving terminal. In addition, a signal with a low spreading factor can degraded by a relatively low power interferer signal, as the l
Khayrallah Ali
Wang Yi-Pin Eric
Chin Stephen
Ericsson Inc.
Kim Kevin
Myers Bigel & Sibley & Sajovec
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