Telecommunications – Transmitter and receiver at separate stations – Distortion – noise – or other interference prevention,...
Reexamination Certificate
1999-10-11
2004-11-23
Maung, Nay (Department: 2684)
Telecommunications
Transmitter and receiver at separate stations
Distortion, noise, or other interference prevention,...
C455S025000, C455S138000, C455S273000, C455S562100, C342S368000, C342S379000
Reexamination Certificate
active
06823174
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to wireless communications. More particularly, the present invention relates to adaptive antenna systems.
2. Description of the Related Art
With the advent and proliferation of digital communication systems, the need for high capacity, high performance systems continues to accelerate. These needs have prompted a strong interest in the development of efficient antenna systems for use at a base station. Efficient antenna systems can increase the capacity and performance of existing digital communications systems without modification of the standardized wireless link protocols.
FIG. 1
shows a typical base station
10
and its corresponding coverage area. The coverage area of the base station
10
corresponds to the geographical region over which the base station
10
is capable of servicing a remote unit. For example, within the coverage area of the base station
10
, a series of remote units
12
A-
12
N are shown. The base station
10
is sectored in that it provides three distinct coverage areas
14
A,
14
B and
14
C in a manner typical of modem base stations. In general, a base station comprises three or more sectors dividing the coverage area into 120° or smaller sections to provide a 360° azimuth field. The use of sectors improves the overall performance and capacity of the base station.
Each sector
14
A-
14
C has a separate antenna system. The use of separate systems decreases the interference between remote units located in different sector coverage areas. For example, the remote unit
12
C is within the coverage area
14
B and, therefore, provides very little interference to the remote unit
12
N located within the coverage area
14
C. In contrast, remote units
12
A and
12
B are each located within the coverage area
14
A, therefore, their signals interfere with one another to some extent at the base station
10
.
To reduce the interference created by remote units operating within a common coverage area, a variety of multiple access schemes have been developed. For example, code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA) or frequency hopping can be used to reduce the interference within a sector. In each of these types of systems, the use of multibeam antenna systems to further sectorize the base station coverage area further reduces co-channel interference and increases the capacity of the system.
For example, to further reduce the interference between remote units within a sector, an antenna array can be used to divide a typical 120° base station sector coverage area into smaller segments called “beams”.
FIGS. 2A and 2B
are graphs showing a typical narrow-beam coverage area pattern in polar and rectangular format, respectively. As shown in
FIGS. 2A and 2B
, in addition to a narrow main beam
20
A, multiple sidelobes
20
B-
20
E are also present. In general, the amplitude of the sidelobes
20
B-
20
E are lower than the main lobe
20
A. For example, in the embodiment illustrated in
FIGS. 2A
2
B, each sidelobe
20
B-
20
E is at least 30 decibels (dB) down from the main lobe
20
A.
FIGS. 3A and 3B
show a top view and a side view of an antenna array capable of producing the coverage area pattern shown in
FIGS. 2A and 2B
. Each of the three antenna arrays
24
A-
24
C is made up of eight array elements
26
A-
26
H. Together the three antenna arrays
24
A-
24
C provide a full 360° coverage area. In
FIG. 3B
, the eight array elements
26
A-
26
H have a nominal one-half wavelength spacing.
FIG. 3C
is a block diagram showing additional circuitry coupled to the antenna array
24
A which make up a beamformer capable of producing the coverage area pattern shown in
FIGS. 2A and 2B
. The output of each array element
26
A-
26
H is coupled to a weighting block
28
A-
28
H, respectively. The weighting blocks
28
A-
28
H provide amplitude tapering and phase shifting, thus, effectively multiplying the incoming signals by a complex set of weights, {W
m
, m=1 . . . 8}. (Through out this text, complex functions and numbers are denoted by underscored text.) The outputs of the weighting blocks
28
A-
28
H are summed in a summer
30
. Weighting the output of each array element
26
A-
26
H by the weighting blocks
28
A-
28
H controls the gain at the peak of the beam, the width of the beam and the relative gain of the sidelobes.
Each array element
26
A-
26
H within the antenna array
24
A ideally has an identical pattern gain and shape over the field of view of the array. This pattern, called the element factor, typically varies as the function of the angle from the normal to the array face. In typical systems, the antenna array comprises 8 or 16 array elements (i.e., m=8 or m=16) and associated weighting blocks. The weighting blocks shown in
FIG. 3C
are sufficient to create one narrow beam such as shown in FIG.
2
A. To create additional beams, additional weighting blocks and summers must be used.
Referring again to the example of
FIG. 2A
, if a remote unit
22
A is located within the main lobe
20
A and a remote unit
22
B is located within the sidelobe
20
, the base station receives the signal energy transmitted by both the remote unit
22
A and
22
B. Although the signal from the remote unit
22
B is reduced by the gain of the sidelobe relative to the main beam, the signal from the remote unit
22
B may still cause significant interference with the signal from the remote unit
22
A.
In the prior art, adaptive antenna techniques have been used to change the coverage area pattern when the remote unit signal within a sidelobe is interfering with the signals in the main beam. These adaptive antenna techniques detect the presence of an interfering signal and modify the coverage area pattern generated by the antenna beamformer to further suppress the interfering signals in the sidelobes. For example, in the situation shown in
FIG. 2A
, it would be advantageous to decrease the size of or place a null in the sidelobe
20
E so that the effects of signal from the remote unit
22
B on the signal from remote unit
22
A may be reduced. Prior art has proposed many of these “smart antenna array” designs to achieve this purpose, but in general, their complexity makes their implementation costly and limits their use in standard terrestrial wireless systems.
In the case shown in
FIG. 2A
, a null can be placed within the sidelobe
20
E to decrease the effects of the signal from the remote unit
22
B on the system. However, placement of a null within a sidelobe produces a corresponding increase in sidelobe-gain at some other location as illustrated in FIG.
2
C. In
FIG. 2C
, nulls have been place at approximately −60,−40,20,38 and 60 degrees from boresight. Notice that the sidelobe having a peak at approximately 28 degrees from boresight has a maximum gain that is greater than −20 dB with respect to the gain of the main lobe. In fact, it is possible for the gain of a sidelobe to exceed the gain of the main lobe if certain weighting parameters are selected.
FIG. 4
is a block diagram showing an adaptive null steering system which is also known in the art as a coherent sidelobe cancellation antenna system. The system includes an antenna array
40
which operates in a similar manner to the system shown in FIG.
3
C. For example, the antenna array
40
can be configured to produce a standard narrow beam such as the antenna pattern shown in FIG.
2
B. The antenna pattern includes the sidelobes
20
B-
20
C as shown. In addition, the antenna system in
FIG. 4
comprises two auxiliary antennas
42
A and
42
B. The antennas
42
A and
42
B are coupled to complex weighting blocks
44
A and
44
B, respectively. The values D
1
and D
2
within the elements
44
A and
44
B, respectively, are complex weights which can be set to form an auxiliary antenna pattern. For example, an antenna pattern
82
in
FIG. 5
represents an antenna pattern for the auxiliary antennas
42
A and
42
B. Note that the antenna
Dell-Imagine Robert A.
Masenten Wesley K.
Ditrans IP, Inc.
Maung Nay
Sobutka Philip J.
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