mon implementation in which the entire SS band is partitioned into N = WSSIR equally spaced FH tones; these are then grouped into Nb = N/M adjacent, non-overlapping M- ary bands, each with bandwidth MR. Under this arrangement, the PN binary k-tuples direct the frequency synthesizer to any of Nb = 2k carrier fre- M - ARY BAND Figure 2.3. Frequency structures for FH/MFSK systems with N tones equally separated by R: in (a), M-ary bands are contiguous and non- overlapping, whereas in (b), these bands are shifted by R which scrambles the FH tone/M-ary symbol mapping (M = 4 here). Non-Coherent Frequency-Hopped Systems quencies, and each FH tone is assigned to a specific, hop-invariant M-ary symbol. It is conceivable that a sophisticated jammer could exploit this fixed assignment scheme. One method of scrambling the FH tone/M-ary symbol mapping from hop to hop is to allow the synthesizer to hop the carrier over all but M - 1 of the N available frequencies SO that adjacent M-ary bands are only shifted by R as shown in Figure 2.3(b). A more jam-resistant (and more expensive) approach is to use M distinct frequency synthesizers to indi- vidually hop the M-ary symbols, destroying the contiguous nature of an M- ary band. There are situations in which it is desirable to avoid certain regions of the radio frequency (RF) band (e.g., fading or narrowband jamming), and here FH enjoys a distinct advantage over DS systems. The synthesizer algorithm which maps the k-chip PN segments into specific carrier frequencies can be modified to eliminate these undesirable bands, resulting in a discontinuous spectrum. In addition to its anti-jam (AJ) capability, an SS signal is generally diffi- cult to detect and even harder to decipher by an unauthorized receiver: this characteristic is usually referred to as a low probability of intercept (LPI). Most interceptors operate as energy detectors, and they have to monitor the received signal long enough to achieve a sufficiently high signal-to-noise ratio (SNR) for reliable detection in the presen