Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1998-11-25
2003-04-01
Kizou, Hassan (Department: 2662)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S337000, C370S342000, C370S347000, C455S422100, C375S146000, C375S147000
Reexamination Certificate
active
06542485
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to communication systems, and more particularly to wireless communication systems such as code division multiple access (CDMA) systems for fixed wireless loop (FWL) and other applications.
BACKGROUND OF THE INVENTION
FIG. 1
shows a portion of a conventional omni-beam FWL system
10
. The portion of system
10
shown includes four hexagonal cells
12
-
1
,
12
-
2
,
12
-
3
and
12
-
4
, each with a corresponding base station
14
-
1
,
14
-
2
,
14
-
3
and
14
-
4
, and a subscriber unit
16
. The system
10
will generally include numerous additional cells, base stations and subscriber units configured in a similar manner. It is assumed in this system that the base stations are equipped with omni-directional antennas, and that the positions of the subscriber units are fixed. The base station
14
-
3
of
FIG. 1
is in communication with the subscriber unit
16
in cell
12
-
3
, e.g., for providing a communication channel for an on-going voice or data call. The omni-beam FWL system
10
may be configured using a number of different techniques.
FIG. 2
shows an example of how the omni-beam FWL system
10
may be implemented using a time division multiple access (TDMA) technique such as that used in the Digital European Cordless Telephone (DECT) standard. In accordance with this TDMA technique, different frequencies are used for the different cells, such that among the cells, users are separated in frequency. A suitable frequency reuse pattern, e.g., a seven-cell hexagonal reuse pattern, may also be used in order to limit the number of different frequencies required. Within a given cell, users are separated in time through the use of a sequence of time slots
20
, including time slots
22
-
1
,
22
-
2
, . . .
22
-N. The system
10
may also be implemented using a code division multiple access (CDMA) technique. In accordance with this technique, the same frequencies but different codes are used for each of the cells, such that the codes are used to separate users in different cells and within a given cell. Some frequency separation may also be used in conjunction with the code separation in order to reduce interference from other cells. Additional details regarding conventional CDMA systems are described in, for example, Andrew J. Viterbi, “CDMA: Principles of Spread Spectrum Communication,” Addison-Wesley, 1995, which is incorporated by reference herein. Other conventional CDMA systems are described in, for example, TIA/EIA/IS-95A, “Mobile Station—Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” June 1996, and ANSI J-STD-008, “Personal Station—Base Station Compatibility Requirements for 1.8 to 2.0 GHz Code Division Multiple Access (CDMA) Personal Communication Systems,” both of which are incorporated by reference herein.
FIG.3
shows a conventional narrow-beam FWL system
30
. The portion of system
30
shown includes four hexagonal cells
32
-
1
,
32
-
2
,
32
-
3
and
32
-
4
, each with a corresponding base station
34
-
1
,
34
-
2
,
34
-
3
and
34
-
4
. In this system, it is again assumed that the positions of the subscriber units are fixed. The base stations in system
30
are equipped with directional antennas which generate narrow beams
36
. At any given time, only a subset of the total number of beams in the system is active, i.e., communicating with users. The beams
36
are made as narrow as possible in order to target only a single user, and thereby minimizing inter-cell interference. In order to provide an increased capacity, the system
30
may be configured such that all cells use the same frequencies, i.e., a frequency reuse factor of 1.
FIG. 4
shows an alternative implementation in which a given cell
42
-i includes nine electronically-steerable narrow beams
46
. The beams
46
are separated into three sectors, each including three beams designated
1
,
2
and
3
. This provides a more manageable hopping pattern, e.g., turning on a designated single beam within each sector at any given time.
FIGS. 5 and 6
illustrate the difference between sectorization and steerable beams in a narrow-beam system such as system
30
of
FIG. 3
, which assumes a frequency reuse factor of 1.
FIG. 5
shows a pair of sectorized cells
50
-
1
and
50
-
2
having base stations
52
-
1
and
52
-
2
, respectively. In this example, a beam
53
from one of six sectors of the cell
50
-
1
and a beam
55
from one of the six sectors of the cell
50
-
2
will generate co-channel, i.e., inter-cell, interference. If the beams are sectorized but not steerable, then it is generally not possible to mitigate this type of co-channel interference adaptively unless the sectors are separated in frequency.
FIG. 6
shows an arrangement in which a pair of cells
60
-
1
and
60
-
2
, via respective base stations
62
-
1
and
62
-
2
, generate sectorized and steerable beams. It can be seen that, as illustrated by the relative positions of steerable beams
63
and
65
, that such an arrangement can be used to provide adaptive mitigation of co-channel interference.
FIG. 7
illustrates a conventional technique for separating uplink (UL) and downlink (DL) traffic for a given antenna beam in an omni-beam or narrow-beam system. In this technique, an uplink channel
72
U
and a downlink channel
72
D
are separated in frequency as shown, i.e., frequency division duplexing (FDD) is used to separate uplink and downlink traffic. Users of the uplink and downlink channels
72
U
and
72
D
are separated in time, using sequences of time slots
74
-
1
,
74
-
2
,
74
-
3
. . . and
76
-
1
,
76
-
2
,
76
-
3
. . . , respectively.
The conventional techniques described above suffer from a number of disadvantages. For example, it is generally very difficult to generate narrow beams targeted to single users, as in the narrow-beam FWL system
30
of FIG.
3
. In addition, narrow beams of this type are susceptible to increased interference from effects such as shadowing and problematic sidelobes. Use of narrow beams in conjunction with a TDMA technique within a given cell can lead to catastrophic interference. For example, if beams from adjacent cells overlap, there is catastrophic interference since the signals are neither separated in frequency nor in time among the different cells, but are instead separated in the spatial domain. In a high density environment, this limitation can severely restrict capacity. Another problem is that conventional FDD techniques, such as those used to separate uplink and downlink in
FIG. 7
, generally cannot adaptively tradeoff capacity between uplink and downlink. As a result, these FDD techniques are generally not well suited for use with, e.g., data-oriented wireless services. It is apparent from the foregoing that further improvements are needed in wireless communication techniques in order to overcome these and other problems of the prior art.
SUMMARY OF THE INVENTION
The invention provides apparatus and methods for wireless communication in fixed wireless loop (FWL) and other types of systems in which, e.g., information is communicated in a given cell of the system between subscriber units and a base station over an uplink and a downlink. In accordance with a first aspect of the invention, a code division duplex (CDD) time-slotted CDMA wireless communication system is provided. Communications on the uplink are separated from communications on the downlink using code division duplexing, and communications with different subscriber units in the cell are separated using a code division multiple access technique, e.g., time-slotted CDMA. The code division duplexing may be implemented by, e.g., assigning a first subset of a set of codes to the uplink and a second subset of the set of codes to the downlink. The code assignment process may be repeated for different time slots, such that the number of codes in the first and second subsets varies across the time slots in accordance with uplink and downlink traffic demands. The system may utilize electronically-steered
Hoang Thai D
Kizou Hassan
Lucent Technologies - Inc.
Ryan & Mason & Lewis, LLP
LandOfFree
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