JED - May 2009 - (Page 53)

EW101 Communications EW – Part 24 Jamming LPI Signals I n the next few columns, we will discuss techniques for jamming low-probability-of-intercept (LPI) communications. LPI signals were discussed in the May 1998 through July 1998 “EW 101” columns. Other related columns that you might find profitable to review are the October 2007 EW 101 and November 2007 EW 101 columns on digital communication, and the April 2008 EW 101 through October 2008 EW 101 columns on the location of communications emitters. LPI COMMUNICATION SIGNALS Signals associated with LPI communications have special modulations designed to make them difficult for normal types of receivers to detect. Ideally, a hostile receiver will not even be able to determine that such a signal is present. This is accomplished by spreading the frequency range over the LPI signal that is broadcast. Thus, the signals also are called PROCESSING GAIN spread-spectrum signals. As shown in Figure 1, a special secRemoving the spreading modulation from an LPI signal is ond modulation is applied to this type of signal to spread its said to create a processing gain. This refers to the fact that spectrum. Three types of spreading modulations are used: the spread signal has a very low signal-to-noise ratio when • Frequency hopping, in which the transmitter periodically received by a normal receiver. After despreading, the received hops to a pseudo-randomly selected frequency. The hopsignal has a significantly higher signal-to-noise ratio. However, ping range is much greater than the bandwidth of the signal carrying the information being communicated (i.e., information the information bandwidth). input modulator • Chirp, in which the transmitter is rapidly tuned across a frequency range spreading that is significantly wider than the spreading demodulator information bandwidth. modulator • Direct Sequence Spread Spectrum, in which the signal is digitized at a rate information output much higher than required to carry demodulator the information, thereby spreading the energy of the signal across a wide Figure 1: LPI communication systems add special frequency spreading modulations for bandwidth. transmission security. There also are LPI signals in which more than one of the above spreading techniques is employed. despread output The spreading demodulator (in the spreading signal with with receiver) shown in Figure 1 must be demodulator LPI spreading processing gain synchronized with the spreading modulator (in the transmitter) to reverse the spreading modulation (see Figure 2). This jamming signals do not synchronization returns the signal to the same bandwidth have synchronized spreading with spreading and thus are not despread that it had before spreading. We call this modulator the information bandwidth. The synchronization requires that both the modulator Figure 2: Synchronization with the spreading modulator allows the spreading modulation to be and demodulator be controlled by the same removed from the intended received signal but not from jamming signals. pseudo-random function, based on a digital code sequence. In addition, the code in the receiver must be in phase with the transmitter code. This requires a synchronization procedure at system start-up, and also at any time the receiver or transmitter has been out of communication for an extended time. Except for the synchronization requirements, the spreading/ despreading process is transparent to the people or computers passing information from the transmitting location to the receiving location. In some circumstances, synchronization requires a delay before transmission can begin. We will discuss each of the frequency spreading techniques in separate columns that also describe the techniques used to jam them. Note that we will have a later series of columns discussing the generation and uses of codes. The Journal of Electronic Defense | May 2009 53

Table of Contents for the Digital Edition of JED - May 2009

JED - May 2009
The View From Here
From the President
The Monitor
Washington Report
World Report
Protecting Helicopters
Ground-Based COMINT Steps Up
Roost Profile
EW 101
AOC News
Index to Advertisers
JED Sales Offices
JED Quick Look

JED - May 2009