Piecewise coherent beamforming for satellite communications

Multiplex communications – Communication over free space – Repeater

Reexamination Certificate

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Details

C455S013100, C455S013300

Reexamination Certificate

active

06240072

ABSTRACT:

BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates generally to satellite communication systems. More particularly, the present invention relates to a ground based beamforming system designed to provide flexible frequency and beam allocation.
II. Description of the Related Art
Fundamentals of Analog Beamforming
The term beamforming relates to the function performed by a device in which energy radiated by an aperture antenna is focused along a specific direction in space. The objective is either preferentially receiving a signal from a particular direction or preferentially transmitting a signal in a particular direction.
For example, in a parabolic antenna system, the dish is the beamforming network since it takes the energy that lies within the aperture formed by the perimeter of the dish and focuses it onto the antenna feed. The dish and feed operate as a spatial integrator.
Energy from a far-field source, which is assumed to be aligned with the antenna's preferred direction, arrives at the feed temporarily aligned and is summed coherently. In general, sources in other directions arrive at the feed unaligned and are added incoherently. For this reason, beamforming is often referred to as spatial filtering.
Beamforming may also be carried out using phased-array antennas. An array can be considered as a sampled aperture. When an array is illuminated by a source, samples of the source's wavefront are recorded at the location of the antenna elements.
The outputs from antenna elements may be subjected to various forms of signal processing. In these cases, phase or amplitude adjustments are made to produce outputs that provide concurrent angular information for signals arriving from several different directions in space. When the outputs of the elements of an array are combined via some passive phasing network, the phasing will usually arrange for the output of all the elements to add coherently for a given direction. If information were desired regarding signals arriving from a different region in space, another phasing network would have to be implemented.
The network that controls the phases and amplitudes of the excitation current is typically called the beamforming network. If beamforming is carried out at a radio frequency (RF), the analog beamforming network typically consists of devices such as phase shifters and power dividers that adjust the amplitudes and phases of the elemental signals to form a desired beam. The beamforming network can be implemented using microwave lenses, waveguides, transmission lines, printed microwave circuits, and hybrids.
It is sometimes desirable to form multiple beams that are offset by finite angles from each other. The design of a multiple-beam beamforming network, known as a beamforming matrix, is much more complicated than that of a single-beam, beamforming network. In a beamforming matrix, an array of hybrid junctions and fixed-phase shifters are used to achieve desired results.
Fundamentals of Digital Beamforming
Beamforming functions can be achieved in the digital domain, especially when the original signal is in digital form (e.g., signals from a digital radio). Digital Beamforming is a combination of antenna technology and digital technology. A generic, digital beamforming antenna system is comprised of three major components: an antenna array, a digital transceiver, and a digital signal processor (DSP).
In a digital beamforming antenna system, the received signals are digitized at the element level. Digital beamforming is based on capturing the radio frequency (RF) signals at each of the antenna elements and converting them into two streams of binary baseband signals known as the in-phase (I) and quadrature-phase (Q) channels.
Included within the digital baseband signals are the amplitudes and phases of signals received at each element of the array. The beamforming is accomplished by weighting these digital signals, thereby adjusting their amplitudes and phases, such that when added together they form the desired beam. This function, usually performed using an analog beamforming network, can be done by a special purpose DSP.
The key to this technology is that the receivers must all be closely matched in amplitude and phase. A calibration process that adjusts the values of the data stream prior to beamforming accomplishes the matching.
One advantage of digital beamforming over conventional phased arrays is the greatly added flexibility without any attendant degradation in the signal-to-noise ratio (SNR). Additional advantages include:
1. Beams can be assigned to individual users, thereby assuring that all links operate with maximum gain;
2. Adaptive beamforming can be easily implemented to improve the system capacity by suppressing co-channel interference and can be used to enhance system immunity to multipath fading;
3. Digital beamforming systems are capable of performing, in the digital domain, real-time calibration of the antenna system. Thus, variations in amplitude and phase between transceivers can be corrected in real time; and
4. Digital beamforming has the potential for providing a major advantage when used in satellite communications. If, after the launch of a satellite, it is found that the satellite's capabilities or performance of the beamformer needs to be upgraded, a new suite of software can be telemetered to the satellite. Digital beamforming allows both the direction and shape to be changed by changing the coefficients in the multiplications performed by the digital signal processor. Analog beamforming has both fixed by hardware component values that are not easily changed.
Digital Beamforming in Satellite Communications
In microwave communication systems, such as those used in communication satellites, networks generate antenna beam signals. These antenna beam signals are then used to drive transmit arrays that in turn form the transmit beams that send communication signals to the intended destination.
Beamforming techniques were introduced to generate electronically steerable and reconfigurable beams. Electronic antenna steering negated many of the disadvantages of mechanical steering in which an antenna was moved mechanically by either rotating itself or the entire satellite at a slow rate. This method only allowed users in a small area to be concurrently served.
Electronic antenna steering, however, provides the ability to focus on many larger areas concurrently with high gain. By controlling the phase and amplitude of the transmit signals fed onto the components of the transmit (feeder) array, the beam direction, shape, sidelobe characteristics, and the Effective Isotropic Radiation Power (EIRP), can be manipulated to the requirement of a particular application. The EIRP, as is well known in the art, is the product of the input power to the antenna and its maximum gain.
Early types of beamforming networks were comprised of a single periodic delay line and only a single beam was formed at a time. Thus, long physical lines were needed to achieve a proper delay condition.
Newer types of beamforming applications employ resonant circuit delay networks to achieve frequency addressability of antenna beams. In the digital domain, the beamforming network is translated into a complex-valued beamforming matrix.
Beamforming can be performed either on the ground or on the satellite. There are several advantages with having beamforming performed on the ground. First, a ground beamforming system releases some satellite load while allowing more beamformers to be placed on the ground than could be placed in a satellite. Second, a beamformer on the ground provides more flexibility for future tuning and modification if a failure or error occurs. Finally, the number of ground-based beamformers can be increased if the need arises. This option is not available if the satellite has already been launched.
The capacity of a satellite system is dependent on the number of times the allocated frequency spectrum can be reused. Polarization and spatial isolation of beams have been used to in

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