Anti-Jam One mechanism to support anti-jam on a commercial product as opposed to relying solely on purpose-built frequency hop spread spectrum systems is to couple the coding gains of direct sequence spread spectrum (DSSS) with the channel-hopping capabilities native to a time division multiple access (TDMA) very small aperture terminal (VSAT) system. In research and testing of anti-jam technologies, iDirect Government has been impressed with the potential of signal excision for highly sensitive defense, homeland security, government and first responder communications use. First, it is beneficial to define the term Signal Excision. Signal Excision is any technology, either analog or digital which can identify and remove an interfering signal without the need for additional bandwidth. This is in contrast to the traditional method of Anti-Jam that relies on spread spectrum which of course, requires a large spread factor and thus enormous amounts of bandwidth to overcome even small jamming signals. Employing a signal excision anti-jam as a first line of defense against interference can be very beneficial. Quite often, there simply isn’t enough available bandwidth in theatre to overcome a threat. There are a number of Signal Excision technologies on the market today. Analog filter manufactures have been building active filters for years. In the digital domain Successive Interference Cancelation (SIC) is a common method used for signal

2 Anti-Jam White Paper Version 1 – May 2020

excision. One limitation of SIC is in order to be effective the interfering signal must be at a significantly higher or lower power from the signal of interest. If the interfering signal is a Carrier Wave (CW), the difference in power between the signal of interest and the interferer is typically on the order of four to five dB. If the interfering signal is a modulated carrier the power delta required between the two signals for SIC to be effective is even higher. A signal excision technology known as Communication Signal Interference Remover or CSIR had come to my attention at the Satellite Show in 2018. CSIR showed a great deal of promise and was a more mature technology than what had been marketed two years ago. There were a number very impressive and very useful characteristics of CSIR. One impressive CSIR feature is in order to be effective the power difference between the Signal of Interest and the interferer could be very small. When thinking of threat assessments many people think in terms of either very large modulated carriers or huge CWs. Quite often, in either blue on blue or red on blue interference the interferer power level is close to that of the signal of interest. This type of threat necessitates the use of a signal excision methodology which can differentiate small differences in power. Another key differentiator of signal excision systems is their ability to remove a wide range of interferer types. In the real world we see a number of types of interferers including Carrier Waves, modulated carriers, sweeping tones, multiple CWs/combs, hopping carriers and at times a combination of multiple threats. Having tested CSIR we have found it to be effective against all of these interferers. Another substantial benefit is the relative compactness of the FPGA image. This makes the CSIR technology particularly portable to existing FPGA based radio systems. Of course, no signal excision system can adequately address all jam threats. We have found the CSIR implementation of signal excision to reach its limits when the interfering signal is a modulated carrier whose symbol rate is a significant percentage of the signal of interest. For example, if the signal of interest is QPSK modulated 10 MHz and the interferer is QPSK 2.5 MHz, the CSIR signal excision will provide excellent headroom and coding gains to overcome the interferer. As the interferer’s symbol rate approaches that of the signal of interest the signal excision will no longer be effective and another, more brute force anti-jam methodology such as spread spectrum will be required. The exact point where signal excision stops being effective will be determined by a number of factors, very importantly the forward error correction being employed. There are two broad categories of spread spectrum, frequency hop and direct sequence. Historically, frequency hop spread spectrum has been employed to address jamming threats though both methods provide the same gains for a given spread rate. Interestingly, we are discovering the coding gains realized when CSIR and direct sequence spread spectrum (DSSS) are used in tandem far exceed a simple addition of the two separate gains. This combination of CSIR and DSSS leads to a highly efficient Anti-Jam solution. There are other benefits to employing a DSSS anti-jam. A DSSS waveform is inherently compatible with existing TDMA VSAT architectures. Given the DoD drive toward leveraging COTS products and avoiding expensive, often disappointing customized developments, augmenting existing TDMA infrastructure with DSSS and CSIR would be a boon for all. A combination of anti-jam techniques seems to yield the greatest benefits and enables the use of cost effective and proven technologies. Certainly, the state of the art is not static. As analog filters, DSP and waveforms improve so will anti-jam technology. The state of the art must continue to evolve as jamming threats become more common place and in the case of red on blue the adversaries become more sophisticated.