JED - January 2012 - (Page 39)

EW 101 Spectrum Warfare – Part 9 Digital Communication continued By Dave Adamy The Journal of Electronic Defense | January 2012 A digital waveform cannot be directly transmitted; it must be modulated onto an RF carrier using one of several modulations. PAM Some of the modulations carry one bit per transmitted baud, and some carry mulZERO ONE ONE tiple bits per baud. The choice of modulation impacts the amount of bandwidth required to carry a given number of bits per second of information, and the FSK percentage of bit errors that will be caused by the signalZERO ONE ONE to-noise ratio in the transmitting link. This discussion will be covered in two parts (this month and next). Figure 1 shows three of the waveforms that carry one OOK bit per baud. These are pulse amplitude modulation (PAM), ZERO ONE ONE frequency shift keying (FSK) and on/off keying (OOK). PAM generates one modulating amplitude for a 1 and another for Figure 1: Digital information can be carried by a number of modulations, a 0. FSK carries a 1 at one frequency and a 0 at another including pulse-amplitude modulation, frequency-shift keying and frequency. OOK is shown with a signal present for a digital on/off keying. Each has a unique modulation condition for a 1 and another for a 0. 1 and no signal for a 0; these can be reversed. Generally, the bandwidth required to send one of these codes is 0.88 x the bit rate. This is the width of the frequency spectrum of the moduindicates its phase. The arrow rotates counter clockwise 360 lated signal 3 dB down from the peak of the curve shown in Figure degrees during each RF cycle of the transmitted signal. In this 5 of the September 2011 “EW 101” column. case, the phases shown are relative to a reference signal. Figure 2 shows two waveforms that carry digital information by phase modulating a carrier. Binary phase shift keying (BPSK) is shown with a zero phase shift when a 1 is carried and a BPSK 1 0 180-degree phase shift when a 0 is carOne bit 180º 0º per baud ried. These can be reversed. Quadrature phase shift keying (QPSK) has four deZERO ONE ONE fined phases, 90 degrees apart. Each of Phase these phase conditions defines two bits 90º of information. As shown in the figure, a 0-degree phase shift indicates a “zero, 01 zero” digital signal, 90 degrees indicates a “zero, one” signal, and so on. ObviQPSK 00 0º 180º Two bits ously, any two binary values can be as10 ONE ZERO ONE ZERO per baud signed to any of the four phase states. Phase 11 Also shown in this figure are “signal ONE ONE ZERO ZERO vector diagrams” for each of these mod270º ulations. In a signal vector diagram, the Figure 2: Two common digital modulations carry information in the phase of the transmitted length of the arrow indicates the signal signal. Binary phase shift keying has two phase positions and one bit per transmitted baud. amplitude, and the angle of the arrow Quadrature phase shift keying has four phase positions and two bits per transmitted baud. 39

Table of Contents for the Digital Edition of JED - January 2012

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
Washington Report
World Report
US IRCM Market Forecast
USMC Moves Out on Air-Land EW
Book Review
EW 101
AOC News
Views from the AOC Convention
Index of Advertisers
JED Quick Look

JED - January 2012