FIELD OF THE INVENTION

The invention relates to antennas and, more particularly, to antennas which generate and steer communications beams.

BACKGROUND OF THE INVENTION

In a high bandwidth communications system where the communications nodes are in motion relative to earth-based subscriber units, a subscriber unit typically maintains a link with the moving communications node using a narrow communications beam. A narrow communications beam allows the earth-based subscriber unit to transmit information to and receive information from the moving communications node at high data rates. Typically, a more narrow receive or transmit beam allows a higher data rate to be used between the communications node and the earth-based subscriber.

Previous earth-based systems used for tracking moving communications nodes, such as low earth orbit satellites, involve the use of mechanically steered reflector antennas. However, when the communications node is a low earth orbit satellite, the satellite may travel from one horizon to another and be in view of the subscriber unit for only a few short minutes. Therefore, since the mechanically steered reflector antenna must constantly be moved in order to maintain the communications link between the satellite and the subscriber unit, the mechanical components begin to wear and must periodically be replaced. This periodic replacement increases the life cycle cost which an earth-based subscriber must pay in order to receive and transmit high-bandwidth information to and from a moving satellite communications node.

Some other techniques for maintaining a communications link with a moving communications node involve the use of two-dimensional electronically scanned antenna arrays through the use of a digital beamformer. In a two-dimensional array which uses a digital beamformer, each transmit antenna element incorporates an individual power amplifier. Additionally, each receive element incorporates an individual low noise amplifier. The need for individual amplification of both receive and transmit antenna elements, as well as the need to perform a large number of digital operations in the beamformer itself, as well as the need for interconnections between the beamformer and the array of antenna elements involves substantial complexity in the required electronics and is therefore cost prohibitive for use by individual earth-based subscribers.

Therefore, what is desirable, is a low-cost system with minimal moving parts to provide beam steering in the communications antenna of the subscriber unit. A low-cost beam steering communications antenna using fewer moving parts also increases the reliability of the antenna over complex mechanically steered systems. These features make communications with a moving satellite accessible to a greater number of users with increased reliability.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is pointed out with particularity in the appended claims. However, a more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the figures, wherein like reference numbers refer to similar items throughout the figures, and:

FIG. 1

is a block diagram and illustrates a ground based hybrid antenna system in communications contact with moving communications nodes in accordance with a preferred embodiment of the invention;

FIG. 2

is a block diagram and illustrates a hybrid antenna system which provides communications with moving communications nodes in accordance with a preferred embodiment of the invention;

FIG. 3

illustrates a cross-sectional view of a hybrid antenna system employing Barium Strontium Titanate voltage controlled dielectric phase shift elements in accordance with a preferred embodiment of the invention;

FIG. 4

illustrates a cross-sectional view of another hybrid antenna system employing micro-electromechanical systems (MEMS) switches as phase shift elements in accordance with a preferred embodiment of the invention; and

FIG. 5

is a flow chart and illustrates a method of steering a communications beam using a digital beamformer and plurality of phase shift elements in accordance with a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

A low-cost system for beam steering in a communications antenna provides the capability for subscribers to receive and transmit high bandwidth information to and from moving satellite communications nodes. The system combines low-cost equipment which can be mass produced using semiconductor processes in order to provide a highly reliable and robust antenna which can establish and maintain a communications link with a moving communications node. Additionally, the use of two such hybrid antenna systems integrated into the same package enables a smooth hand-over of communications with one moving node to communications with a second moving node. Therefore, terrestrial-based users can maintain uninterrupted contact with the satellite communications network as each satellite comes within view. As each satellite nears the horizon, or becomes masked by foliage or other obstructions, a second communications beam can generated in order to establish a link with the second moving node which is within view of the antenna system. Furthermore, in the event that a moving communications node or other space-based emitter generates interference, the antenna system can minimize this interference by generating a null in the appropriate direction.

FIG. 1

is a block diagram and illustrates a ground based hybrid antenna system in communications contact with moving communications nodes in accordance with a preferred embodiment of the invention. In

FIG. 1

, satellite communications nodes

10

and

15

are in communications with earth-based subscriber unit

20

through communications beams

25

and

30

, respectively. In a preferred embodiment, these communications nodes are representative of a global satellite network with an interface to a terrestrial voice and data infrastructure. Additionally, satellite communications nodes

10

and

15

can communicate with each other and other similar satellites through intersatellite cross-links. Thus, satellites

10

and

15

provide voice and data capabilities which enable earth-based subscriber unit

20

to transmit data to and receive data from the terrestrial voice and data infrastructure through satellite communications nodes

10

and

15

.

In

FIG. 1

, satellite communications nodes

10

and

15

are in motion relative to earth-based subscriber unit

20

. By way of example, and not by way of limitation, satellite communications node

15

is moving away from earth-based subscriber unit

20

and will soon pass beyond the horizon and out view of subscriber unit

20

. Meanwhile, satellite communications node

10

is also in view of earth-based subscriber unit

20

and will soon be directly overhead of earth-based subscriber unit

20

. In a preferred embodiment, earth-based subscriber unit

20

maintains a link with satellite communications nodes

10

and

15

as these satellites move relative to the surface of the earth

40

. Each of satellite communications nodes

10

and

15

may originate from different points on the horizon as well as terminate at different points on the horizon. Thus, satellite communications node

10

may come into view of earth-based subscriber unit

20

from a direction of due North while satellite communications node

15

may come into view from a direction of North by Northeast. Further, satellite communications node

10

may terminate at a horizon location of due South while satellite communications node

15

may terminate at a horizon direction of South by Southwest.

In a preferred embodiment, earth-based subscriber unit

20

employs a “make before break” technique in which the communications link with satellite communications node

15

is maintained until a link with satellite communications node

10

can be established. Thus, only after a link with satellite communications node

10

has been established is the link with satellite communications node

15

discontinued. Consequently, earth-based subscriber unit

20

includes two independently steerable antennas in order to facilitate this capability.

FIG. 2

is a block diagram and illustrates a hybrid antenna system which provides communications with moving communications nodes in accordance with a preferred embodiment of the invention. In

FIG. 2

, processors

205

and

206

control the operations of digital beamforming networks

210

and

211

. Additionally, processors

205

and

206

control the operations of control networks

221

and

222

. In a preferred embodiment, processors

205

and

206

each maintain a record of the current locations of satellite communications nodes

10

and

15

of FIG.

1

. Processors

205

and

206

command digital beamforming networks

210

and

211

as well as control networks

221

and

222

in order to adjust receive and transmit communications beams to the locations of satellite communications nodes

10

and

15

. Processors

205

and

206

can also maintain a record of the locations of other satellites similar to satellite communications nodes

10

and

15

which are part of the global communications network. Further, processors

205

and

206

may also maintain a record of the locations of other satellite communications nodes which could interfere with transmissions from satellite communications nodes

10

and

15

. This allows processors

205

and

206

to determine if a null or other minimum gain point of a communications beam should be directed toward the source of the interference in order to mitigate the effects of the interference on the communications.

In a preferred embodiment, digital beamforming networks

210

and

211

provide beam steering in a first dimension while control networks

221

and

222

provide beam steering in a second, and preferably orthogonal, dimension. Therefore, digital beamforming networks

210

and

211

may provide beam steering in a North South direction while control networks

221

and

222

provide beam steering in an East West direction. In the example of

FIG. 2

, each output of digital beamforming networks

210

and

211

provides beam steering commands which control a particular column of antenna elements

240

and

241

. Thus, for this example, the complexity of each of digital beamforming networks

210

and

211

is driven only by the number of rows of antenna elements

240

and

241

.

Digital beamforming networks

210

and

211

are coupled to digital to analog converters

215

and

216

, respectively. Digital to analog converters

215

and

216

function to convert the digital inputs from digital beamforming networks

210

and

211

to analog waveforms. The analog waveforms from digital to analog converters

215

and

216

are conveyed to up converters

217

and

218

, respectively. Up converters

217

and

218

function to convert the analog outputs of digital to analog converters

215

and

216

to carrier signals to that can be radiated by antenna elements

240

and

241

.

The carrier signals from up converters

217

and

218

are input to distributing elements

219

and

220

, respectively. In a preferred embodiment, distributing elements

219

and

220

convert an input from up converters

217

and

218

into a group of outputs. In a preferred embodiment, distributing elements

219

and

220

apply a weighting factor to each output. This allows each output to form the basis of an antenna radiation pattern in a dimension which is orthogonal to the dimension controlled by digital beamforming networks

210

and

211

.

The outputs of distributing elements

219

and

220

are then coupled to phase shift elements

230

and

231

, respectively. Phase shift elements

230

and

231

function to adjust the phase of the amplitude tapered outputs from distributing elements

219

and

220

so that an antenna radiation pattern can be generated in a dimension which is preferably orthogonal to the dimension controlled by digital beamforming networks

210

and

211

. In a preferred embodiment, control networks

221

and

222

control the amount of phase shifting applied to each of phase shift elements

230

and

231

. Through this control and occasional modification of phase, the resulting antenna radiation pattern can be steered to the desired location in the orthogonal dimension.

The outputs of phase shift elements

230

and

231

are coupled to antenna elements

240

. In a preferred embodiment, antenna elements

240

and

241

are arranged in a two dimensional array. Antenna elements

240

and

241

can be any type of radiating elements such as a dipole, monopole above a ground plane, patch, or any other type of conductive element in which an electromagnetic wave is launched in response to an electrical current being generated on a conductive surface. Additionally, antenna elements

240

and

241

can comprise a waveguide slot or other type of radiating element which produces an electromagnetic wave as a function of an electric field being generated within the waveguide slot. Finally, antenna elements

240

and

241

can comprise a microstrip element which produces an electromagnetic wave as a function of a change in impedance caused by a notch or other indentation made in the microstrip transmission line.

Although

FIG. 2

describes the elements which are desirable for synthesizing a transmit communications beam, a receive communications beam can be generated using reciprocal system hardware. For the case of generating a receive communications beam, a group of low noise amplifiers are preferably inserted in series with each of antenna elements

240

and

241

. The amplified signals from antenna elements

240

and

241

are phase shifted by way of control networks

221

and

222

and combined by way of distributing element

219

and

220

which are preferably linear, two way devices. In an alternate embodiment, low noise amplifiers are placed at the output of distributing elements

219

and

220

so that only the combined signal is amplified. This is advantageous since the number of low noise amplifiers is reduced from an amount equal to the number of antenna elements

240

and

241

to an amount equal the number of columns of the antenna elements.

The resultant combined receive signals are down converted by way of down converters which are inserted in place of up converters

217

and

218

. The down converted signals are input to analog to digital converters which are preferably inserted in place of digital to analog converters

215

and

216

. The resultant digital inputs are then conveyed to a receive digital beam forming networks which are similar to digital beam forming networks

210

and

211

.

FIG. 3

illustrates a cross-sectional view of a portion of a hybrid antenna system (

300

) employing barium strontium titanate voltage controlled dielectric phase shift elements in accordance with a preferred embodiment of the invention. The structure of

FIG. 3

(

300

) is repeated for each row of antenna elements

310

which comprise the antenna system. Antenna elements

310

are similar to antenna elements

240

or

241

of FIG.

2

.

In

FIG. 3

, waveguide

340

is used as a distributing element which performs the function of distributing element

219

of FIG.

2

. Carrier signal inputs are coupled from waveguide

340

into barium strontium titanate media

360

. Although a barium strontium titanate phase shift element is used in the example of

FIG. 3

, other ferroelectric media which exhibit variable dielectric properties as a function of a control voltage applied across a section of the dielectric media can be used. In a preferred embodiment, coupling slots

350

are cut into a wall of waveguide

340

and barium strontium titanate media is in intimate contact with waveguide

340

. The size of each of coupling slots

350

and the position of each slot on the wall of waveguide

340

determine the amount of carrier signal energy coupled from waveguide

340

into barium strontium titanate media

360

. Although this embodiment makes use of a waveguide and coupling slots, these are provided by way of example, and not by limitation. Other transmission lines structures, such as microstrip or stripline, as well as with other coupling techniques, such as microstrip coupled lines, can also be used to perform the function of distributed element

219

or

220

of FIG.

2

.

The carrier signal energy from each of coupling slots

350

is then propagated through barium strontium titanate media

360

. As known to those skilled in the art, barium strontium titanate possesses a physical property of a changing dielectric constant in response to a voltage applied across anode

320

and cathode

330

. Although not shown in

FIG. 3

, anode

320

and cathode

330

are connected to a control network such as one of control networks

221

and

222

of

FIG. 2. A

control signal in the form of an analog voltage from the control networks applied across anode

320

and cathode

330

functions to change the phase of the carrier signal traveling through barium strontium titanate media

360

.

The phase shifted carrier signal output is coupled to one of antenna elements

310

. The lower conductive side of each of antenna elements

310

is in intimate contact with barium strontium titanate media

360

. Thus, the incoming carrier signal from the barium strontium titanate media excites a current on the upper surface of each of antenna elements

310

. This, in turn, causes an electromagnetic signal to be radiated from the upper surface of each of antenna elements

310

. The radiated energy from each of antenna elements interferes constructively and destructively at specific angles in front of the antenna system of

FIG. 3

, thus producing the desired antenna radiation pattern in the dimension along the “Z” axis of

FIG. 3

which is steerable in the “Y” axis.

Although described as a transmit antenna, the reciprocal nature of the antenna of

FIG. 3

allows the antenna to generate a receive communication beam as well as a transmit communications beam.

FIG. 4

illustrates a cross-sectional view of a section of another hybrid antenna system (

400

) employing micro-electromechanical systems (MEMS) switches as phase shift elements in accordance with a preferred embodiment of the invention. The structure of

FIG. 4

(

400

) is repeated for each row of antenna elements

310

which comprise the antenna system. Antenna elements

410

are similar to antenna elements

240

or

241

of FIG.

2

.

In

FIG. 4

, coupling probes

450

extend into waveguide

440

. The placement of coupling probes

450

on the wall of waveguide

440

controls the amount of energy coupled from waveguide

440

into the coupling probe. Each coupling probe conveys carrier signal energy to one of MEMS switch groups

460

. Although not shown in

FIG. 4

, each MEMS switch group is controlled by a discrete voltage from a control network such as one of control networks

221

and

222

of FIG.

2

.

In a preferred embodiment, a connection to a control network allows MEMS switch groups

460

to switch in and switch out sections of transmission line in the carrier signal path from waveguide

440

to antenna elements

410

. Through this change in the length of the carrier signal path, the relative phase of each signal coupled to antenna elements

410

can be controlled. In a preferred embodiment, each MEMS switch group includes a loaded line microstrip phase shifter including eight switches in order to provide four-bit phase resolution of 22.5 degrees. However, a greater or lesser number of MEMS switches may be employed according to the phase resolution requirements of the particular application.

The phase shifted carrier signal output from each MEMS switch is coupled to a matching layer in order to couple a maximum amount of carrier signal energy to each one of antenna elements

410

. As the carrier signal is coupled to each of antenna elements

410

, an electromagnetic signal is radiated from the upper surface of each of antenna elements

410

. The radiated energy from each of antenna elements interferes constructively and destructively at specific angles in front of the antenna system of

FIG. 4

, thus producing the desired antenna radiation pattern in the dimension along the “Z” axis and steerable in the “Y” dimension of FIG.

4

.

Although described as a transmit antenna, the reciprocal nature of the antenna of

FIG. 4

allows the antenna to generate a receive communication beam as well as a transmit communications beam.

FIG. 5

is a flow chart and illustrates a method of steering a communications beam using a digital beamformer and plurality of phase shift elements in accordance with a preferred embodiment of the invention. The antenna system of

FIG. 2

is suitable for performing the invention. The method begins at step

510

with a plurality of antenna excitation signals being generated using a digital beamforming network. Step

510

includes a summation of a plurality of antenna element signals from each digitally generated beam multiplied by a plurality of amplitude weighting functions to form a plurality of digital representations of amplitude and phase of the antenna excitation signals.

In step

520

, antenna excitation signals from the output of the digital beamforming network are converted to analog waveforms to create analog representations of antenna excitation signals which are up converted in step

530

. In step

540

, the amplitude and phase of certain ones of the antenna excitation signal are shifted in order to produce amplitude and phase shifted antenna excitation signals.

In step

550

, the amplitude and phase shifted antenna excitation signals are coupled to an antenna array allowing information to be transmitted to or received from a satellite communications node. In step

560

, the quality of the communications link is evaluated in order to determine if any steering adjustments to the beam need to be performed. In the event that the link between the satellite communications node and the antenna system is acceptable, the method waits for a predetermined period of time, as in step

570

. After this time has expired, the method returns to step

560

where the link quality is again evaluated.

In the event that the link quality evaluation of step

560

determines that the link with the satellite communications node is degraded, the method returns to step

510

where the communications beam is adjusted. By repeating steps

510

through

560

, a robust link with a moving satellite communications node can be maintained.

A method similar to that of

FIG. 5

can be envisioned for the antenna of

FIG. 2

generating a receive communications beam. In this embodiment, the method begins with coupling signals transmitted from an external source to the antenna array elements. In the next step, the amplitude and phase of each of the received signals are modified and combined. The method continues with a down conversion of the receive signals, followed by a conversion from an analog representation to a digital representation of each signal. In the final step of the method, the digital representation of each signal is fed to a digital beamforming network.

A low-cost system for beam steering in a communications antenna provides the capability for subscribers to receive and transmit high bandwidth information to and from a moving communications node. The system combines low-cost equipment operated with minimal or no moving parts in order to provide a highly reliable antenna which can communicate with a moving communications node. Additionally, the use of two hybrid antenna systems enables a smooth hand-over from communications with one moving node to communications with a second moving node. Therefore, users can maintain contact with the satellite communications system without interruption. Furthermore, in the event that a moving communications node generates interference, the antenna can minimize interference from the interfering satellites by generating a null in the appropriate direction. For these reasons and others, the system represents a significant advancement in satellite communications technology by providing the general public with the capability to receive satellite communications services at a minimal cost.

Accordingly, it is intended by the appended claims to cover all modifications of the invention that fall within the true spirit and scope of the invention.