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Channel and Interference adaptive SATCOM Digital beam-former


OBJECTIVE: Develop an adaptive digital beamformer that can adapt to the RF channel and interference conditions for optimal throughput and/or reception quality. DESCRIPTION: A beam forming system uses an array of antenna feeds to form a desired composite antenna pattern. This is accomplished by weighting (amplitude and phase shifting) each individual feed and then summing the weighted feeds. An algorithm is used to derive an optimal set of weights to form a composite pattern which may, for example, electronically"steer"to a desired source signal, or null an undesired interferer. To date, most satellite uplink beamformers apply their weights to the RF signals received by the feeds (usually after a low-noise amplification stage) [1]. Following weight combining, downconversion and filtering are performed to 1) isolate a particular frequency band containing the signal of interest, and 2) reduce the bandwidth of the signal in preparation for analog-to-digital conversion. A drawback of this approach is that the weights are frequency-independent and therefore, very little opportunity exists for shaping the composite antenna pattern over frequency. For example, if a user is located at angle ? and frequency f1 while an undesired interference exists at ? and f2 then we would like an antenna pattern with a main lobe at (?,f1) and a null at (?,f2). However, with frequency-independent weights, the pattern is necessarily also frequency independent. Some systems, such as AEHF, overcome this limitation by"sharing"the beam. In such a system, a TDMA scheme is employed in which, for a given time slot, the weights are optimized for one user. The composite beam is steered to the user and bandpass filters isolate the user from interference. On the next time slot, the beam steers to the next user. However, in such a system, the achievable throughput is degraded by the amount of beam sharing. If the beam forming could be done digitally, then a filter-and-sum [1] approach could be followed in which the weights are replaced by linear filters. This would allow for composite patterns that vary with both spatial direction and frequency. Such patterns could enable all users to communicate with the satellite simultaneously so long as they maintain separation in space or frequency. Such implementations were not feasible in the past as they require wideband ADC"s which can digitize the entire satellite band. This type of digital beamformer could achieve a significant improvement in throughput versus a beam shared system. In fact, the throughput could potentially increase by a factor of M where M is the number of users sharing the beam. The optimal weights for a given user/interference scenario must be derived. Since the digital implementation gives the processor access to all feeds and all frequencies, an adaptive algorithm is a likely method for deriving optimal weights. The large amount of information available to the processor opens up many possibilities for weight update algorithms. This SBIR is seeking a SATCOM digital beamformer technology and implementation that is adaptive to the channel and interferences. The proposed beamformer should demonstrate adaptation of the beamformer weights to optimize for various impairments experienced in the satellite channel such as interference, jammers, non-ideal antenna frequency dependent characteristics, and fading. Moreover, the implementation should demonstrate new capabilities in enhancing throughput for example by simultaneously receiving multiple channels with improved reception quality. The proposed system should be agnostic to the RF frequency, bandwidth, number of digital bits, and modulation types. It should be able to operate in the same vibration, temp, and radiation environment as the existing WGS and AEHF SATCOM terminals. The simulation model should include a high-fidelity SATCOM channel model that accounts for atmospheric condition, and various interference and jamming conditions. The simulation performance should quantify the beamformer performance that is adaptive to the changing atmospheric and interference conditions. The simulation model should be bit-exact and is ready for the FPGA implementation. This research on the improved beam forming technology will ultimately find utilization within the commercial satellite communication systems to provide enhanced reception and reliability of voice, video, and data transmissions via commercial satellites. PHASE I: Investigate architecture and algorithms to determine the feasibility of implementing an adaptive beamformer for satellite communications via mathematical analysis and model and simulation. Investigate the feasibility of the proposed beamformer to operate in the comm-on-the-move environments. PHASE II: Develop and demonstrate a prototype adaptive beamformer for satellite communications. The demonstration system should integrate the adaptive beamformer hardware with the COTS components to form a complete SATCOM terminal. For the demonstration purpose, the Ka-band WGS (30-31 GHz uplink and 20.2-21.2 Ghz downlink), and the SHF-band AEHF (44GHz uplink and 20GHz downlink) should be used as examples. PHASE III: Integrate the prototype unit to demonstrate its capability in beamforming for various military systems such as WGS and AEHF. Improved beam forming technology will ultimately find utilization within commercial satellite communication systems. REFERENCES: 1. L. Wang and D. Ferguson, WGS Air-Interface for AISR Missions, IEEE 2007 MILCOM. 2. D. H. Johnson and D. E. Dudgeon, Array Signal Processing: Concepts and Techniques, Prentice-Hall, Upper Saddle River, NJ, 1993. 3. on Digital beam forming.
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