Communications: directive radio wave systems and devices (e.g. – Directive – Including a steerable array
Reexamination Certificate
2000-05-03
2003-12-23
Issing, Gregory C. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a steerable array
C455S562100
Reexamination Certificate
active
06667714
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to techniques for controlling the downtilt angle of phased-array antennas, such as those used in the base stations of wireless communication networks.
2. Description of the Related Art
In a conventional wireless communication network, communications with wireless units (e.g., mobile telephones) are supported by base stations, each configured with one or more antennas that provide communication coverage over an area surrounding the base station referred to as the base station cell. A typical base station cell may be divided into (e.g., three) sectors, with different antennas configured to support communications for the different sectors. In order to provide a relatively large cell size, base station antennas are typically configured at a higher height (e.g., on the tops of transmission towers) than the wireless units located within that cell. In order to communicate with wireless units located anywhere within a base station cell, including right next to the base station itself, base station antennas are typically configured with a downtilt angle to “point” the antennas down to provide the appropriate coverage.
One way to configure an antenna with a downtilt angle is to physically mount the antenna pointing at an angle below horizontal. Another way to achieve a downtilt angle is to use a phased-array antenna that can be pointed “electrically” by selecting appropriate phase shifts at the various antenna elements in the array.
FIG. 1
shows a block diagram of a conventional N-element, parallel-fed, fixed-phase, phased-array antenna
100
. Antenna
100
comprises a power splitter
102
, N phase shifters
104
, each phase shifter configured with a corresponding antenna element
106
, where the N phase shifters
104
are configured in parallel to power splitter
102
. Power splitter
102
receives an RF signal and distributes that RF signal to the N phase shifters
104
(e.g., splitting the signal power equally or in a shaped (e.g., cosine) manner among the different phase shifters). Each phase shifter
104
i
shifts the phase of its received portion of the RF signal by a particular fixed phase-shift angle &phgr;
i
and passes the resulting phase-shifted RF signal to its corresponding antenna element
106
i
, which radiates that phase-shifted portion of the RF signal as a wireless electromagnetic (E-M) signal.
If the phase-shift angles &phgr; at the N phase shifters
104
are selected appropriately, the resulting composite radiated E-M signal from the entire antenna array will form a uniform wavefront that propagates in a particular direction. As depicted in
FIG. 1
, to achieve a particular downtilt angle &agr;, the element array of antenna
100
is configured with a progressive phase shift such that the phase-shift angle &phgr;
i
applied by each phase shifter
104
i
increases linearly from the first phase shifter
104
1
through the N
th
phase shifter
104
N
.
In general, the greater the number of antenna elements in the array, the more accurately and well-defined can be the coverage area (or footprint) of the antenna. This can be very important, especially in applications such as wireless communication systems, where base stations need to be distributed over a geographic area and configured with antennas that provide precise antenna footprints to ensure complete coverage over that geographic area with some overlap in adjacent antenna footprints to support handoffs for mobile wireless units, yet not with too much overlap in order to avoid undesirable interference between the signals of different wireless units.
Although
FIG. 1
shows antenna
100
configured to transmit RF signals, antenna
100
can also be configured to receive RF signals, either at the same time as, or instead of, being configured to transmit RF signals, in which case, power splitter
102
(also) functions as a power combiner.
For relatively large downtilt angles and large arrays (e.g., more than four elements), the phase-shift angle &phgr;
i
for the last few phase shifters
104
i
, where i=N, N−1, . . . , can become very large. This is not a problem for fixed-angle arrays. However, since the heights of base station antennas may vary from cell to cell, and the sizes of cells may vary from base station to base station, the magnitude of the downtilt angle will also typically vary from cell to cell. Moreover, the desired antenna footprint for a particular base station antenna may also vary over time, for example, as more base stations are configured within an existing covered geographic area. As such, it is not always practical to design base station antenna arrays with a fixed downtilt angle.
FIG. 2
shows a block diagram of a conventional N-element, parallel-fed, variable-phase, phased-array antenna
200
. Like antenna
100
of
FIG. 1
, antenna
200
comprises a power splitter
202
, N phase shifters
204
, each with a corresponding antenna element
206
, where the N phase shifters
204
are configured in parallel to power splitter
202
. In antenna
200
, however, the N phase shifters
204
are configured as part of a phase-shifter assembly
208
, which is configured to a motor
210
, which is in turn configured to a controller
212
.
Controller
212
receives phase control signals that determine how to control the operations of motor
210
, which in turn drives phase-shifter assembly
208
. Phase-shifter assembly
208
is typically a mechanical device with movable components (as driven by motor
210
) whose movements affect the electro-magnetic characteristics (e.g., line length) of the various phase shifters
204
to change the magnitude of the phase-shift angle &phgr;
i
applied by each phase shifter
204
i
in a controlled manner.
Because the downtilt angle can be varied in a controllable manner, a single antenna design can be used for different base stations having different antenna heights that require different and varying downtilt angles. One advantage of parallel-fed, variable-phase antennas, such as antenna
200
, is that they can be implemented with minimum insertion phase (i.e., phase difference) between adjacent antenna elements. For example, if the progressive phase shift needs to be 17 degrees in order to achieve a downtilt angle &agr; of 4 degrees, then this can be achieved using parallel-fed phase shifters, where the difference in phase-shift angle &phgr; between adjacent antenna elements
206
i
and
206
i+1
is simply (&phgr;
i+1
−&phgr;
i
)=17°.
Because the insertion phase can be minimized, parallel-fed, phased-array antennas can have relatively wide bandwidths. Typical wireless communication networks use different frequency bands for uplink (i.e., wireless unit to base station) and downlink (i.e., base station to wireless unit) communications. If the bandwidth of parallel-fed, phased-array antennas can be large enough, a single antenna array may be able to support both the uplink and downlink frequency bands. In that case, a single phased-array antenna can be used to both transmit downlink signals to the wireless units and receive uplink signals from the wireless units.
Unfortunately, for large ranges in downtilt angle (e.g., greater than 4 degrees) and large arrays (e.g., more than eight elements), the last few phase shifters (e.g.,
204
N
,
204
N−1
, . . .) of parallel-fed antenna
200
can become impractical to realize, because those phase shifters must be able to provide a relatively large range of phase-shift angles &phgr; (e.g., from as small as 0 degrees for a zero downtilt angle to as large as 180 degrees for a downtilt angle of 4 degrees). In order to avoid this problem, series-fed phased-array antennas are typically used.
FIG. 3
shows a block diagram of a conventional N-element, series-fed, variable-phase, phased-array antenna
300
. Like antenna
200
of
FIG. 2
, antenna
300
comprises a power splitter
302
, a phase-shifter assembly
308
with N phase shifters
304
, each with a corresponding antenna element
306
, a motor
310
that drives ph
Issing Gregory C.
Lucent Technologies - Inc.
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