JED - August 2011 - (Page 48)
Spectrum Warfare – Part 4
By Dave Adamy
The Journal of Electronic Defense | August 2011
hether intentional or unintentional, interfering signals reduce the fidelity of received information. We will discuss modulation and coding techniques to reduce the impact of interference.
SPREADING THE TRANSMITTED SPECTRUM
Spread spectrum techniques were discussed in detail in the May to July 1998 “EW 101” columns and were revisited in the context of communication jamming in the May through October 2009 columns. This discussion is focused on the transfer of information vs. bandwidth and the nature of the interference environment. The description, “Low Probability of Intercept” (LPI), is also used to define these signals, but because this deals with only one advantage of the signals, we will talk about them as spread-spectrum (SS) signals. In general, these signals have a much wider transmission spectrum than required to carry the transmitted information. The despreading of the signal at the receiver recovers the information transmitted while providing a “processing gain” that increases the ratio of the recovered information to the false outputs from received interference. Note that all of these types of systems trade noise/interference reduction for increased transmission bandwidth requirement. A simple way to think about this is to consider commercial frequency modulated (FM) broadcast signals.
Figure 1: An FM signal carries information as variations in the transmitted frequency.
Figure 2: The transmitted FM signal carries its information in a bandwidth, which is determined by the selected modulation index.
The output SNR improvement formula (in dB) is: SNR = RFSNR + 5 + 20 log β Where: SNR is the output signal to noise ratio in dB RFSNR is the pre-detection signal-to-noise ratio in dB In order to achieve this SNR improvement, the RFSNR must be above a threshold level: either 4 or 12 dB, depending on the type of demodulator used in the receiver. For commercial FM broadcast signals, the maximum modulating frequency is 15 kHz, and the modulation index is 5. With the most common type of demodulator, the RFSNR threshold is 12 dB. Thus, the broadcast bandwidth is 150 kHz (which is 2 x 15 kHz x 5). With a minimum threshold signal out of the receiving antenna for the most common type of demodulator, the output SNR is 31 dB (which is 12 + 5 + 20 log 5 = 12 + 5 + 14). The frequency modulation, improved the output SNR by 19 dB. Note that pre-emphasis (increasing the power of higher modulating frequencies) in the transmitter and de-emphasis (decreasing the power of higher modulating frequencies) in the receiver can allow a few more dB of SNR improve-
COMMERCIAL FM BROADCAST
The frequency modulated signal was the first widely used spread-spectrum technique. Figure 1 shows the modulation. Wideband FM improves signal quality by increasing the signal-tonoise ratio (SNR) and signal-to-interference ratio as a function of the square of the amount by which it spreads the transmission bandwidth. The spreading ratio is called the modulation index. It is the ratio between the maximum frequency offset from the carrier and the highest modulating frequency as shown in Figure 2. The cost of this SNR improvement is that transmission requires additional bandwidth. Commercial FM frequency assignments are 100 kHz apart and there must be multiple channel slots between occupied channels in a geographic area. With large modulation index, the transmission bandwidth is: BW = 2 fm β Where: BW is the transmitted bandwidth fm is the maximum frequency of the information signal β is the FM modulation index
Table of Contents for the Digital Edition of JED - August 2011
The View From Here
From the President
Defining a Career Path in EW
Technology Survey:SIGINT/DF Antennas
AOC Member Page
Index of Advertisers
JED Quick Look
JED - August 2011