artial-band multitone jamming. Hence, for each form of coherent data modulation considered, we shall direct all of our attention to these two jammer types, developing in each case the performance corre- sponding to the worst case jamming strategy. Although the primary emphasis of this chapter is on pure FH quadrature modulation systems, later on we shall consider the additional improvement in jam resistance offered by superimposing a balanced pseudonoise (PN) modulation on the individual hop frequencies. This hybrid SS technique is referred to as FH/PN modulation and is normally employed in situations where the system requires anti-jam protection beyond that which FH or PN modulation is capable of producing on its own. 1.1 PERFORMANCE OF FH/QPSK IN THE PRESENCE OF PARTIAL-BAND MULTITONE JAMMING An FH/QPSK signal is characterized by transmitting (Figure 1.1) (1.1) in the i-th signalling interval 1)TtiTs, where w(i) is the particular carrier radian frequency selected by the frequency hopper for this inteval.² According to the designated SS code, (ii) is the information symbol which ranges over the set of possible values m=1,3,5,7 (1.2) and S is the transmitted average power. 2We assume here the case of slow frequency hopping (SFH), i.e., the hop rate is equal to or a submultiple of the information symbol rate and that the frequency hopper and symbol clock are synchronous. Thus, in a given symbol interval, the signal frequency is constant and the jam- mer, if transmitting a tone at that frequency, affects the entire symbol interval. Performance of FH/QPSK in the Presence of Partial-Band Multitone Jamming 1/25 sin wo Spreading Code QPSK Information Symbol d(1) Coherent Phase Modulator Frequency s(i)(+) Hopper Figure 1.1. Block diagram of a coherent FH/QPSK modulator. At the receiver (Figure 1.2), the sum of additive white Gaussian noise n(t), the jammer J(t), and a random phase-shifted version of the transmitted sig- nal s(4)(t; 0) are first frequency dehopped, then coherently demodulated by a conventional QPSK demod