 case case for hard yields better perofrmance than the hard 3.6 UNCODED FREQUENCY-HOPPED BINARY FREOLENCY-SUIFT-KEYING basic modulation technique and changing the carrier approach frequency to gener- Taking pseudorandom a manner is the frequency-hopping most common modulations used modula- with some ating a spread-spectrum signal. M-ary The frequency-shift-keying (MFSK) illustrates some frequency tions together hoping with are non-coherent the reception. This section Basic Concepts and System Models known Channel State Hard Decision p = 1 10-7 -2 O .05 2 4 6 8 EB/N, 10 (dB) 12 14 16 18 3.21. Repeat code m = 5 with known jammer state/hard decision. Figure basic concepts with the frequency-hopped signals. binary frequency- shift-keying additional (FH/BFSK) spread-spectrum the form Ordinary BFSK signals have + (3.71) + n = integer. where Here Tb is the data bit time and {dn} are the independent data bits (3.72) with probability 1/2 dn .={-1. = 1, with probability 1/2 (3.73) Typically we choose A wTb== Uncoded Frequency-Hopped Binary Frequency-Shift-Keying Known Jommer State Hard Decision Eb/Nj (dB) Figure 3.22. Repeat code m=9 with known jammer state/hard decision. SO that the two possible transmitter tones are orthogonal for all relative phase shifts over the T second interval. Frequency hopping of this BFSK signal is done with a pseudorandom result- binary sequence that is used to select a set of carrier frequency shifts ing in the frequency-hopped signal + d Awt] n = integer (3.74) where is the particular hop frequency chosen for the n-th transmission interval. Wn Generally, if L pseudorandom binary symbols are used to select a frequency shift each T seconds, then there are at most 2L distinct frequency shifts shift values possible. The range of values taken by these frequency this total defines the total spread-spectrum signal bandwidth Wss. Although Basic Concepts and System Models spread bandwidth need not be contiguous, it is assumed here that this band is a contiguous frequency range. Figure 3.23 illustrates