Tuesday, February 23, 2016

Mitigating Cosite Interference in UAVs

by Doug King dking(at)polezero.com

Military radios must be able to operate in severe cosite interference environments (Figure 1.1 defines cosite interference). Cosite interference is a problem faced by many RF and microwave communications platforms; including Unmanned Systems. Military radios often operate in close proximity to additional radios, giving rise to cosite interference. The following article explains the issues associated with military radios operating in close proximity to additional interferers and how Tunable Filters are utilized in real-time applications. Finally, MPG-Pole/Zero’s recent advances in mitigating cosite interference are summarized.

Issues associated with military radios operating in close proximity to additional interferers: 
Multiple transmitters coupled to antennas in close proximity create a condition called reverse intermodulation, characterized by the coupling of energy from one transmitter into the antenna of another, creating a simultaneous flow of reverse and forward energy. Coupled energy mixes in the nonlinearities in the output network of the transmitter to create an infinite number of intermodulation products. The products are then re-propagated to the collocated receivers, creating products of sufficient level to preclude reception at those frequencies. Thus, a cosite transmitter’s output carrier signal can significantly degrade the performance of the receiver.

How Tunable Filters are utilized in cosite interference applications: 
The use of a receive filter or filter/LNA cascade such as that introduced in the transmit chain can create “preselection” of the energy from the receive antenna and reduce the relative level of the cosite interferer to the desired signal. Under this condition, the debilitating effect of cosite interference is mitigated by the selectivity of the preselector

As in the transmit environment, nonlinear effects in the receive chain can be the source of additional cosite interference. The preselection filter serves to minimize the level of the interfering signals prior to the receive nonlinearity, thereby minimizing any resulting products created within the receiver. Pole/Zero designs and tests the filters and LNAs that comprise the cascade filter to ensure that acceptable levels of distortion occur under these conditions.

Greater isolation can effectively be achieved through the use of selective filtering at the transmitter to minimize broadband noise. Selective filtering is applied following the primary noise sources in the transmit signal chain, having the overall effect of lowering the broadband noise without necessitating an increase in antenna isolation.

For greater selectivity, multiple filters can be placed in cascade with low noise amplifiers (LNAs) for inter-filter isolation and filter loss recovery purposes, followed by a power amplifier designed for efficient operation and low noise output. Further reductions in broadband noise and improved immunity to reverse intermodulation distortion can be achieved with the addition of a high power tunable filter at the output of the PA.

Recent MPG-Pole/Zero tunable filter advances: 
MPG-Pole/Zero’s recent tunable filter advances for cosite interference mitigation solutions include:
• Highly integrated filter products with significant SWaP reduction, compared to legacy filters, that maintain 5W in-band power over the entire military tactical radio tuning range in single- and dual-channel configurations;
• Miniature SMT bandpass filter options from 30 MHz to 3GHz;
• Narrowband and wideband interference cancelers, some of which do not require an interferer reference, thereby enabling cancellation of off-platform interferers;
• Deep notch filters to create communications channels in wideband, high power signals;
• Miniature, light-weight filter and power amplifier cascades for cosite interference issues inherent in UAV retransmission applications.

Reprinted with permission from CRUSER NEWS. All opinions expressed are those of the respective author or authors and do not represent the official policy or positions of the Naval Postgraduate School, the United States Navy, or any other government entity. The inclusion of these links does not represent an endorsement of the organization, service, or product.

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