JED - September 2009 - (Page 45)

EW101 Communications EW – Part 28 Jamming Chirp Signals By Dave Adamy A lthough chirp is most often associated with range resolution improvement in radars, it can also be used for anti-jamming protection in communication. Frequency modulation (called “chirp” in this case) creates a processing gain that makes the detection or jamming of signals more difficult. SWEEP PERI0D FREQUENCY SWEEP RANGE The Journal of Electronic Defense | September 2009 PSEUDORANDOM SWEEP START Implementing Chirped Communication There are two ways to implement chirp. One is to linearly sweep a digital signal across a frequency range much greater than its information bandwidth. The second way is to apply the chirp to every bit of a digital signal. Both have processing gain based on sweep range versus the information bandwidth of the signal. In general, the processing gain reduces the effective jamming to signal ratio (J/S) by an equivalent ratio. As discussed below, there are ways to increase the effective J/S against chirp signals. TIME Figure 2: A chirped signal is swept across a large frequency range with pseudo-randomly selected start times for its sweep cycles. This precludes a hostile receiver from synchronizing to the chirp sweep. J/S by overcoming the processing gain advantage of the intended receiver. Note that the chirp may not have a constant sweep rate, but can follow any desired frequency vs. time pattern. Chirp on Each Bit The chirp communication technique discussed in most literature places a chirp modulation on each data bit transmitted and recovers the digital data in the receiver as shown in Figure 3. The chirp can be applied either with a sweeping oscillator or using a surface acoustic wave (SAW) chirp generator. A de-chirping filter in the receiver converts signals with specific chirp characteristics into impulses because it has a linear delay versus frequency characteristic. In effect, the signal is delayed to the end of the chirp period to produce an output impulse. In this figure, an “up-chirp” is applied, so the de-chirp filter must have decreasing delay as the frequency increases. This chirp technique allows the digital data to be carried in two different ways: parallel binary channels or single channel with pulse position diversity. Impulse Freq Wide Linear Sweep Using the approach shown in Figure 1, a digitally modulated IF signal is swept across a frequency range much greater than the bandwidth of the information carried by the signal. This produces a transmitted waveform as shown in Figure 2. Note that the start times of the sweeps are randomly varied to prevent a hostile receiver from synchronizing with them. The intended receiver has a similar circuit with a sweeping oscillator that is synchronized to the transmitter. As noted in an earlier column, the information must be carried in digital form so that it can be transmitted at a faster bit rate during the linear part of the sweep and returned to the constant bit rate in the receiver. Otherwise, there would be significant signal drop-outs that would interfere with communication. Because the data is digital, optimum jamming causes about 33 percent bit error rate in the received signal, so partial band jamming will provide the best practical jamming performance in non-sophisticated jammers. (See the July 2009 “EW 101” column.) If the chirped transmitter has a fixed sweep synchronization pattern or if the jamming signal can be delayed (perhaps using a DRFM) it may be practical to analyze the chirp pattern and match it with a follower jammer. This would provide significantly better DIGITAL INFORMATION 45 Time CHIRP GEN XMTR RCVR DE-CHIRP FILTER Figure 3: When a swept FM is placed on a bit of a digital signal, it can be processed by a matched de-chirp filter to create an impulse. Parallel binary channels In some systems, logical ones cause one chirp direction (perhaps increasing frequency) while logical zeros cause the opposite chirp direction (in this case decreasing frequency). This type of system is shown in Figure 4. The chirp frequency slope is typically linear. In the receiver, each received bit causes an impulse output from the appropriate de-chirping filter. Note that the data stream input in the figure is 1, 0, 1, 1, 0; thus the upchirp filter outputs impulses for the first, third and fourth bits while the downchirp filter outputs impulses for the second and fifth bits. These impulses are converted into logical bits to reproduce the digital input to the transmitter. CARRIER PSEUDO-RANDOM SWEEP SYNCHRONIZATION INFORMATION MODULATOR SWEEPING OSCILLATOR TRANSMITTER Figure 1: Chirp can be applied to a digital data stream to provide antidetection and anti-jam protection.

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

JED - September 2009
Contents
The View From Here
Calendar
From the President
The Monitor
Washington Report
World Report
Defeating Threats to Large Aircraft
The Threat Test Resource Gap
EW 101
AOC News
JED Sales Offices
Index of Advertisers
JED Quick Look

JED - September 2009

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